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The  Morphology  of  the  Diencephalic 
Floor 

A  Contribution  to  the  Study  of  Craniate  Homology 

FREDERICK^TILNEY 

From  the  Department  of  Anatomy,  Columbia  University 


Reprinted  from  THE  JOURNAL  OF  COMPARATIVE  NEUROLOGY, 
Vol.  25,  No.  3,  June,  1915 


Reprinted  from  THE  JOL-R.VAI.  OF  CO.MPARATIVK  NKUHOI.OGY,  VOL.  25,  No,  3 
June,   1915 


THE  MORPHOLOGY  OF  THE   DIENCEPHALIC   FLOOR 

A   CONTRIBUTION   TO   THE    STUDY  OF  CRANIATE   HOMOLOGY 

FREDERICK   TILNEY 

From  the  Department  of  Anatomy,  Columbia  University 

THIRTY  FIGURES 

The  fact  that  the  basal  surface  of  the  brain-stem  is  phy- 
letically  more  constant  in  form  than  the  lateral  walls  or  the  roof 
is  so  obvious  as  scarcely  to  need  mention;  yet  in  some  respects 
it  is  a  fact  of  considerable  significance.  It  seems  to  have  im- 
portance in  estimating  the  homological  values  of  certain  parts 
of  the  basal  region  since  this  region  in  consequence  of  its  greater 
morphological  constancy  should  afford  more  exact  evidence  of 
homology.  This  becomes  especially  clear  when  it  is  considered 
how  plastic  are  the  lateral  walls  of  the  neural  tube,  not  only 
in  their  embryonic  development  but  in  the  adaptive  modifi- 
cations of  which  they  are  capable.  A  similar  plasticity  is  seen 
in  the  roof-plate  as  witnessed  by  the  varied  conformation  of 
the  paraphysis,  habenular  region,  epiphysis,  mesencephalic 
tectum  and  cerebellum. 

The  basal  region,  on  the  other  hand,  is  not  without  its  vari- 
ations; nor  is  it  surprising  that  this  region  should  bear  traces 
of  primitive  characters,  particularly  in  the  interbrain,  where 
the  neural  structures  have  always  maintained  such  intimate 
relations  to  the  stomadaeum,  pituitary  gland  and  branchial 
cavity.  Andriezen  (1),  among  others,  has  brought  this  fact 
out  clearly.  He  observed  in  Ammocoetes,  Amphioxus  and 
Balanoglossus,  as  well  as  in  the  larval  and  adult  forms  of  Ascid- 
ians,  a  small  tubular  aqueduct  of  capillary  lumen  and  lined 
by  ciliated  epithelium,  extending  between  the  mouth  cavity 
and  the  forebrain.  This  bucco-neural  duct,  he  believes,  pro- 
vides a  true  water-vascular  system  for  the  central  nervous 

213 


214  FREDERICK   TILNEY 

tissue.  To  the  collection  of  ganglionic  cells  situated  at  the 
upper  end  of  this  duct  he  attributes  a  function  similar  to  that 
of  the  osphradial  organ  in  mollusca,  thus  bringing  it  into  general 
relation  with  the  olfactory  apparatus.  .  It  is  also  his  opinion 
that  the  hypophysis,  in  this  sense,  was  functionally  active  in 
the  ancestral  vertebrates  and  that  while  the  bucco-neural  duct 
has  been  obliterated  the  sub-neural  or  pituitary  gland  with  the 
collection  of  ganglionic  cells  has  persisted.  Ayers  (2)  is  in  ac- 
cord with  this  theory  when  he  states  that  the  hypophysis  arose 
as  an  organ  of  taste  and  the  inf  undibulum  was  its  nerve.  Further 
evidence  of  this  kind  is  furnished  by  Ganin  (3),  who  was  among 
the  first  to  observe  a  connection  between  the  anterior  extremity 
of  the  embryonic  neural  tube  and  the  branchial  cavity  in  Ascid- 
ians.  Similar  observations  were  made  upon  chordata  by  Kowa- 
levsky  (4),  Ussow  (5),  Julin  (6)  and  von  Kuppfer  (7);  the  latter 
expressing  himself  as  follows:  "Der  Verbindungskanal  zwischen 
Hirn  und  Darm  schlage  ich  vor  als  Canalis  Neurentericus  an- 
terior zu  bezeichen  und  die  Glande  hypophysaire  von  van 
Beneden  und  Julin  ware  wohl  am  einfachsten  als  Neural-druse 
zu  benennen." 

It  seems  probable  in  the  light  of  these  observations  that  such 
variations  as  do  appear  in  the  floor  of  the  interbrain  of  craniates 
are  adaptive  in  their  nature.  This  idea  is  borne  out  by  the  fact 
that  of  all  the  structures  arising  from  the  diencephalic  floor- 
plate  the  infundibular  process  is  the  most  variable.  This  proc- 
ess from  its  early  phases  of  development  in  all  forms  maintains 
close  relation  to  the  stomadaeum,  pituitary  evagination  and 
branchial  cavity.  As  an  adult  structure  its  modifications  are 
numerous. 

In  the  selachian  (Mustelus  laevis;  fig.  1,  A)  the  processus 
infundibuli  projects  caudad  from  the  floor  of  the  third  ventricle; 
it  presents  two  surfaces,  i.e.,  a  ventral  or  pituitary  surface  in  con- 
tact with  the  pituitary  gland,  and  a  dorsal  or  saccular  surface 
which  is  much  convoluted  and  highly  vascular  forming  the 
saccus  vasculosus.  In  the  amphibian  (Rana  pipiens;  fig.  1,  B) 
the  same  general  relations  obtain  and  the  two  characteristic 
surfaces  are  present  except  that  the  dorsal  or  saccular  surface 


THE    DIENCEPHALIC    FLOOR  215 

is  less  convoluted  and  less  vascular,  while  the  ventral  one  has 
increased  in  thickness. 

In  sauropsids,  for  birds  as  well  as  reptiles,  the  infundibular 
process  differs  in  certain  details  from  that  of  the  ichthyopsid 
although  the  general  homology  of  the  structure  in  all  these  forms 
is  discernible.  In  all  three  instances  the  cavity  of  the  third 
ventricle  extends  into  the  infundibular  process.  In  the  sela- 
chian and  amphibian  this  communication  is  not  defined  by 
any  marked  constriction.  The  bird  (Gallus  gallus;  fig.  1,  C) 
on  the  other  hand,  shows  a  distinct  constriction  in  the  region 
where  the  cavity  of  the  general  ventricular  chamber,  passes  over 
into  the  recess  of  the  infundibular  process.  The  process  still 
presents  its  two  characteristic  surfaces:  the  pituitary  surface 
is  in  contact  with  the  pituitary  gland,  while  the  dorsal  or  saccular 
surface  is  much  convoluted  and  non-vascular.  This  surface, 
unlike  that  in  the  frog  and  dog-fish,  is  thick.  The  recess  of  the 
infundibular  process  in  the  bird  as  in  the  other  forms  already 
described  presents  numerous  branching  diverticula. 

The  mammalian  structure  is  characterized  by  a  marked 
change  in  that  the  cavity  of  the  third  ventricle  does  not  extend 
into  the  infundibular  process  which  latter,  in  consequence,  be- 
comes solid  except  for  a  small  proximal  portion  of  its  stem. 
These  conditions  are  shown  in  the  dog  (fig.  1,  E).,  So  far  as 
I  am  able  to  state  at  present  one  family  alone,  the  Felidae, 
departs  from  the  mammalian  type  in  this  respect.  Here  the 
third  ventricle  actually  communicates  with  a  large  recess  in  the 
infundibular  process  by  means  of  a  narrow,  tubular  canal  which 
passes  from  the  ventricle  through  the  stem  of  the  process  (fig.  1, 
D).  The  recess  of  the  infundibular  process  shows  no  branch- 
ing diverticula;  the  walls  which  bound  it  are  thick  and  non- 
vascular,  so  that  from  all  appearances  it  may  be  inferred  that  the 
convoluted  saccular  surface,  so  conspicuous  in  the  selachian, 
amphibian  and  sauropsid,  has  been  replaced  by  a  now  very 
extensive  pituitary  surface.  This  supposition  is  rendered  more 
probable  by  the  fact  that  the  entire  infundibular  process  in  the 
mammal  is  completely  invested  by  the  tissue  of  the  pituitary 
gland.  In  the  anthropoids  and  man  (fig.  1,  F)  the  solidification 

THE   JOURNAL  OF  COMPARATIVE   NEUROLOGY,   VOL.   25,   NO.   3 


216 


FREDERICK   TILNEY 


ANNOTATIONS  FOR  ALL  FIGURES 


/,  Aqueduct  of  Sylvius  25, 

i,  Chiasmatic  process  26, 

8,  Cerebellum  27, 

4,  Chiasm  28, 

5,  Corpus  interpedunculare  29, 

6,  Diverticula  sacci  vasculosi  SO, 

7,  Epiphysis  31, 

8,  Ectoptic  zone  of  Schulte  82, 

9,  Foramen  of  Monro 

10,  Hypophyseal  recess  33, 

11,  Infundibular  stem 

12,  Infundibular  canal  34, 
IS,  Infundibular  process  35, 

14,  Infundibular  process;  saccular  sur-      36, 

face  (saccus  vasculosus)  37, 

15,  Infundibular    process;    pituitary      38, 

surface  39, 

16,  Infundibular  process,  lateral  process      40, 

17,  Infundibular  region 

18,  Infundibular  evagination  41, 

19,  Interoptic  groove  42, 

20,  Lamina  terminalis  43, 

21,  Lateral  process  of  post-chiasmatic      44> 

eminence  (lobus  inferior)  45, 

22,  Lateral  eminence  46, 

23,  Median  post-chiasmatic  groove  47, 

24,  Mid-brain 


Mammillary  region 

Mammillary  recess 

Mammillary  body  (posterior  lobe) 

Neuropore 

Optic  vesicle  or  evagination 

Optic  peduncle 

Optico-infundibular  groove 

Post-chiasmatic    eminence     (lobus 

inferior) 
Post-chiasmatic   recess    (recess   of 

inferior  lobe) 

Post-infundibular  eminence 
Post-infundibular  recess 
Post-infundibular  evagination 
Post-mammillary  evagination 
Prechiasmatic  recess 
Paraphysis 
Recess  of  infundibular  process  or 

of  infundibular  evagination 
Supraoptic  crest 
Supraoptic  recess 
Thalamencephalon 
Telencephalon 

Tuberculum  postero-superius 
Tubercle  of  the  floor  of  Schulte 
Velum  transversum 


Fig.  1  Comparative  series  of  infundibular  region.  A,  dog-fish;  B,  frog; 
C,  fowl;  D,  cat;  E,  dog;  F,  man.  2,  chiasmatic  process;  4,  chiasm;  14,  infundib- 
ular process,  saccular  surface;  15,  infundibular  process,  pituitary  surface; 
23,  median  post-chiasmatic  groove;  27,  mammillary  body  (posterior  lobe);  34, 
post-infundibular  eminence. 


1  B 


1  C 


1  D 


-27 


218  FKEDERICK   TILNEY 

of  the  infundibular  process  and  the  exclusion  from  it  of  any  acces- 
sory recess  of  third  ventricle  have  progressed  to  the  most  extreme 
degree,  for  in  these  forms  even  the  stem  of  the  process  is  solid. 

In  this  way  through  a  series  of  changes  from  the  ichthyopsid 
to  the  mammal  the  evolution  of  the  infundibular  process  may  be 
traced.  In  this  series  the  sauropsid  condition  still  bears  evidence 
of  the  saccus-formation  in  its  apparently  retrograding  saccular 
surface;  the  conditions  in  the  Felidae  carry  this  retrograding 
process  one  step  further  toward  the  general  mammalian  type  of 
infundibular  process  from  which  the  saccular  surface  and  the 
saccus-formation  have  entirely  disappeared. 

It  is  not,  however,  until  the  entire  floor  of  the  ventricle  is 
considered  that  the  significance  of  each  of  its  several  parts  may 
be  ultimately  determined.  These  parts  have  been  designated 
by  many  terms,  several  of  which  have  been  devised  with  the 
intention  of  giving  a  phylogenetic  or  embryological  interpreta- 
tion to  the  structures.  Such,  for  example,  is  the  case  with  the 
part  described  by^  Retzius  (9)  as  the  eminentia  saccularis,  for 
this  term  as  applied  to  mammals  imputes  a  genetic  relationship 
between  the  eminence  so  described  and  the  saccus  vasculosus 
of  fish.  That  such  a  relationship  does  not  actually  exist  can, 
I  think,  be  proved.  For  these  reasons  in  considering  this  region 
of  the  brain  it  seems  advisable  to  employ  such  terms  only  as 
shall  be  morphologically  or  topographically  descriptive.  To 
this  end  the  following  suggestions  are  offered  for  the  structures 
found  upon  the  floor  of  the  third  ventricle,  beginning  at  the  optic 
chiasm  and  proceeding  caudad  to  the  mammillary  bodies: 

1.  The  optic  chiasm. 

2.  The  supraoptic  crest,  a  transverse  ridge  extending  across 
the  dorso-cephalic  surface  of  the  optic  chiasm  and  for  a  short 
distance  upon  the  optic  nerves. 

3.  The  post-chiasmatic  eminence,  a  marked  protuberance  of 
the  floor  immediately  caudad  to  the  chiasm;  this  structure  is 
often  referred  to  as  the  bulbus  infundibuli. 

4.  The  infundibular  process,  an  expanded  appendage  to  the 
floor  connected  with  the  infundibular  bulb  by  the  infundibular 
stem. 


THE    DIENCEPHALIC    FLOOR  219 

5.  The    post-infundibular    eminence,    a    small    irregular    and 
median  protuberance  in  front  of  the  corpora  mammillaria  and 
caudad  to  the  infundibular  bulb. 

6.  The  mammillary  bodies. 

7.  The  lateral  eminences,   a  pair  of  bilateral  protuberances 
situated  one  en  either  side  of  the  post-chiasmatic  eminence. 
Of  these  structures  the  post-chiasmatic,  post-infundibular  and 
lateral  eminences  constitute  the  tuber  cinereum,  while  the  in- 
fundibular process  is  appended  to  the  tuber  by  the  infundibular 
stem.     In  an  earlier  paper  (8)  the  writer  so  interpreted  the  text 
and  figures  published  by  Retzius  (9)  as  to  assign  the  term  emi- 
nentia  saccularis  to  the  post-chiasmatic  eminence.     Upon  further 
investigation,  however,  it  became  obvious  that  Retzius  referred 
to  what  is  here  called  the  post-infundibular  eminence. 

In  the  attempt  to  estimate  the  homological  values  of  the 
structures  in  this  region  of  the  brain,  serial  sections  obtained 
from  the  following  adult  forms  were  studied: 

Squalus  acanthias  Lepus  sylvaticus  Canis  latrans 

Mustelus  laevis  Sphingurus  prehensilis  Canis  familiaris 

Lepidosteus  osseus  Mephitis  mephitica  Genetta  vulgaris 

Rana  pipiens  Odocoelus  hemionus  Felis  domesticus 

Menobranchus  Odocoelus  virginianus  Felis  pardus 

Iguana  tuberculata  Oryx  beatrix  Felis  leo 

Gallus  gallus  Ovis  tragelaphus  Lemur  macaco 

Botaurus  lentiginosus  Ovis  aries  Macacus  cynomolgus 

Didelphys  quica  Castor  canadensis  Nyctipithecus  trivirgatus 

Bradypus  tridactylus  Mirounga  (Macrorhinus  Babuin  cynocephalus 

Dipus  aegypticus  angustirostris)  Hylobates  hoolock 

Dasyprocta  agouti  Nasua  narica  Simia  satyrus 

Mus  decumanus  Ursus  horribilis  Homo 

Studies  of  this  portion  of  the  neuraxis  in  the  gross,  even  with 
the  aid  of  the  binocular,  are  quite  unsatisfactory  because  of  the 
compact  arrangement  of  the  structures  which  must  be  examined, 
and  because  dissection  in  this  region  can  scarcely  be  performed 
without  seriously  disturbing  the  relation  of  the  parts.  For  this 
reason,  the  Born  method  of  reconstruction  was  employed  in  the 
study  of  the  adult  cat,  dog,  rat,  rabbit,  opossum,  common  fowl  and 
dog-fish.  It  was  also  used  in  the  reconstruction  of  models  which 
demonstrate  the  ontogeny  of  the  diencephalic  structures  in  the 
cat,  chick  and  dog-fish. 


220  FKEDERICK   TILNEY 

ELEMENTS   IN  THE   DIENCEPHALIC    FLOOR  OF  THE  ADULT    CAT 

The  lateral  view  of  a  model  reconstructed  to  show  the  left 
side  of  the  floor  of  the  interbrain  in  an  adult  cat  is  reproduced 
in  figure  2.  At  its  cephalic  extremity  is  the  optic  chiasm  (4), 
while  its  most  caudal  structure  is  the  mammillary  body 


13 


Fig.  2  Lateral  view  of  forebrain  reconstruction  in  adult  cat.  X  35.  The 
unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  4,  chiasm;  11, 
infundibular  stem;  18,  infundibular  process;  27,  mammillary  body;  32,  post- 
chiasmatic  eminence;  34,  post-infundibular  eminence;  41,  supra-optic  crest; 
42,  supra-optic  recess. 

Above  the  optic  chiasm  and  extending  from  the  median  line  in 
either  direction  along  the  dorso-cephalic  border  of  the  chiasm 
and  optic  nerve  is  a  ridge-like  elevation,  the  supraoptic  crest  (41)- 
This  ridge  is  most  prominent  at  and  near  the  median  line.  It 
is  most  conspicuous  in  the  carnivores,  especially  in  the  Felidae, 
but  it  is  present  in  all  the  forms  examined.  The  chiasm  (4) 


*        THE    DIENCEPHALIC    FLOOR  221 

forms  a  well  defined  ridge  crossing  the  outer  surface  of  the 
floor,  but  immediately  caudal  to  it  is  the  largest  protuberance  of 
this  region,  the  post-chiasmatic  eminence  or  bulbus  infundibuli 
(32}.  It  is  difficult  to  demonstrate  this  eminence  on  the  actual 
brain,  for  the  reason  that  it  is  almost  entirely  invested  by  a 
portion  of  the  pituitary  gland,  the  pars  tuberalis.  As  a  rule, 
this  protuberance  is  torn  away  with  the  hypophysis  when  the 
attempt  is  made  to  study  the'  structures  in  the  floor  of  the  third 
ventricle  and  such  removal  produces  an  artificial  slit-like  open- 
ing into  the  ventricle  which  has  been  called  the  lura.  The 
post-chiasmatic  eminence  presents  a  long  ventral  surface  which 
slants  caudad  and  ventrad  from  the  optic  chiasm;  it  leaves  the 
general  plane  of  the  floor  at  this  level  and  proceeding  for  a  con- 
siderable distance  in  the  direction  of  the  mammillary  bodies 
reaches  its  greatest  prominence  about  midway  between  these 
bodies  and  the  chiasm.  The  ventral  surface  presents  a  shallow 
furrow  whose  long  axis  is  in  the  median  plane.  This  is  the 
median  post-chiasmatic  groove.  In  this  region  the  neural  tissue 
forming  the  floor  of  the  eminence  is  thin.  Laterad  in  both 
directions  the  neural  tissue  rapidly  increases  in  thickness;  its 
ectal  surface  becoming  convex  forms  two  lateral  processes  of  the 
post-chiasmatic  eminence,  one  on  either  side  of  the  median 
post-chiasmatic  groove  and  each  projecting  free  of  the  adjacent 
basal  surface.  The  dorsal  surface  of  the  post-chiasmatic  emi- 
nence is,  in  the  main,  parallel  with  its  ventral  surface  but  caudally 
it  turns  sharply  upward  to  meet  the  plane  of  the  floor.  Two 
lateral  borders  bound  the  eminence,  becoming  more  prominent 
as  they  are  traced  caudad;  for  about  three-quarters  of  their 
distance  they  are  divergent;  they  then  become  convergent 
caudad  and  as  they  approach  each  other  form  with  the  dorsal 
and  ventral  surfaces  of  the  eminence  a  constricted  stem-like 
prolongation,  the  infundibular  stem  (11)  which  projects  caudad 
to  become  continuous  with  the  expanded  infundibular  process 
(18).  The  infundibular  stem  and  the  infundibular  process  are 
invested  by  the  pars  infundibularis  of  the  pituitary  gland. 

In  all  the  other  mammalian  forms  examined  the  post-chiasmatic 
eminence  is  a  prominent  feature  of  the  diencephalon;  it  maintains 


222  FREDERICK   TILNEY 

its  definite  relations  to  the  pars  tuberalis,  and  appears  with  but 
slight  variations  in  the  same  general  conformation  as  described 
in  the  cat.  The  most  considerable  modifications  in  its  form 
are  seen  in  the  anthropoids  and  man.  In  these  forms  it  does 
not  hold  the  same  relation  to  the  floor  of  the  interbrain  as  in 
the  lower  mammals.  This  change  is  occasioned  by  the  forward 
and  downward  rotation  which  occurs  in  the  hypophysis  as  the 
latter  sinks  into  the  deepened  sella  turcica.  Another  factor 
operative  in  this  change  is  the  foreshortening  of  the  sella  in  man 
and  the  apes  which  further  tends  to  force  the  pituitary  gland 
craniad.  The  rotation  from  the  developmental  standpoint 
seems  to  be  secondary  to  the  increased  depth  in  the  pituitary 
fossa,  for  in  the  five-month  human  fetus,  as  the  writer  has 
previously  shown  (8),  the  post-chiasmatic  eminence  occupies 
a  position  corresponding  in  all  details  to  that  of  the  adult  cat. 
It  is  only  in  the  late  fetal  and  early  post-natal  stages  that  the 
protuberance  undergoes  a  change  in  relations  which  in  effect 
is  the  result  of  a  rotation  of  the  hypophysis  through  90°.  When 
this  is  completed  the  surface  described  in  the  cat  and  other 
mammals  as  ventral  no  longer  presents  a  ventral  inclination 
but  is  turned  craniad,  while  the  eminence  as  a  whole  has  become 
elongated  in  its  long  axis  and  constricted  transversely.  As  a 
result  it  has  a  more  or  less  bulbous  appearance,  a  fact  which  has 
given  rise  to  the  term  bulbus  infundibuli. 

The  most  caudal  structures  entering  into  the  floor  of  the  third 
ventricle  are  the  corpora  mammillaria  (27} ;  the  one  on  the  left 
side  is  shown  in  figure  2.  In  the  adult  cat  these  bodies  are  large 
protuberances  situated  one  on  either  side  of  the  median  line  and 
immediately  cephalad  to  the  posterior  perforated  space.  They 
appear  in  all  the  mammals  studied  and  are  also  present  though 
less  conspicuous  in  sauropsid  forms. 

The  post-infundibular  eminence  (34)  occupies  a  position 
immediately  in  front  of  the  mammillary  bodies.  In  lateral 
view  (fig.  2)  it  does  not  appear  so  prominent  as  the  latter  struc- 
tures nor  has  it  the  sharp  lateral  demarcation  of  the  corpora. 
On  the  other  hand,  it  is  definite  in  all  mammals.  So  far  as  may 
be  stated  upon  the  evidence  of  the  material  examined  it  is  most 


THE    DIENCEPHALIC    FLOOR  223 

conspicuous  in  carnivores.  In  primates  it  is  not  always  well 
marked,  yet  in  all  the  apes  examined  it  was  present.  In  man, 
especially  in  later  adult  life,  considerable  care  may  be  required 
to  detect  it,  although  in  many  instances  it  is  quite  as  evident 
as  in  the  carnivores;  this  is  particularly  true  in  the  brain  of  the 
infant  and  child.  The  eminence  appears  in  ungulates;  at  least 
it  was  observed  in  several  varieties  of  artiodactyla  (sheep, 
mountain  goat,  mule  deer,  aoudad  and  Virginia  deer).  It  is 
prominent  in  the  proboscidea  (Indian  elephant)  and  also  occurs 
in  rodents  and  marsupials.  The  post-infundibular  eminence 
appears  as  a  transverse  ridge  extending  across  the  ventricular 
floor.  It  is  most  prominent  at  and  near  the  median  line;  it 
presents  no  sagittal  division  into  bilateral  halves  and  laterally 
merges  with  the  general  plane  of  the  basal  region.  Its  shape 
is  somewhat  variable;  often  it  is  elongated  cephalo-caudad  and 
it  may  be  asymmetrical.  Its  ventral  surface  may  arise  sharply 
to  the  floor  of  the  ventricle  or  it  may  blend  gradually  with  this 
area.  Its  caudal  surface  usually  rises  abruptly  to  the  ven- 
tricular floor. 

The  developmental  history  of  the  post-chiasmatic  eminence 
shows  that  it  is  partially  constricted  off  from  the  basal  portion 
of  the  interbrain  by  the  growth  of  the  pars  tuberalis  of  the  pitui- 
tary gland.  Two  basal  regions  thus  he  above  the  eminence, 
symmetrically  placed,  one  on  either  side  of  the  median  line. 
In  shape  they  are  roughly  triangular  having  their  bases  turned 
mesad  and  their  apices  projecting  laterally.  The  base  of  each 
triangle  extends  from  the  post-infundibular  eminence  almost 
as  far  forward  as  the  chiasm.  As  they  are  followed  laterad 
each  presents  a  protuberance  which  is  most  pronounced  near 
the  apex  of  the  triangle.  These  protuberances  and  the  basal 
areas  with  which  they  are  in  continuation  constitute  the  lateral 
eminences  (22}. 

The  median  sagittal  view  of  the  model  reconstructed  from  the 
diencephalic  floor  in  the  adult  cat  is  shown  in  figure  3.  This 
view  gives  the  ventricular  recesses  many  of  whose  surface  ex- 
pressions have  already  been  discussed.  Cephalad  to  the  chiasm 
(4)  the  ventricular  cavity  extends  forward  and  slightly  down- 


224 


FREDERICK   TILNEY 


ward  as  the  small  prechiasmatic  recess  (38}  which  corresponds 
to  the  median  portion  of  the  supraoptic  crest  (41}.  This  recess 
is  continued  laterad  as  a  long  canal  extending  for  some  distance 
above  the  optic  nerve,  the  supraoptic  recess  (42).  Both  the 
prechiasmatic  and  supraoptic  recesses  are  present  in  all  the  forms 


Fig.  3  Mesial  view  of  forebrain  reconstruction  in  adult  cat.  X  35.  The 
unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  4,  chiasm;  11,  infundibular  stem;  12,  infundibular  canal;  15,  infundib- 
ular process,  pituitary  surface;  27,  mammillary  body;  32,  post-chiasmatic 
eminence;  S3,  post-chiasmatic  recess;  34,  post-infundibular  eminence;  85,  post- 
infundibular  recess;  38,  pre-chiasmatic  recess;  40,  recess  of  the  infundibular 
process;  41  >  supra-optic  crest;  4%,  supra-optic  recess. 

examined.  Caudal  to  the  prechiasmatic  recess  a  large  prom- 
inence rises  from  the  floor  of  the  ventricle  in  a  position  corre- 
sponding to  the  optic  chiasm.  This  is  the  chiasmatic  process 
(2).  From  the  caudal  extremity  of  this  process  the  floor  falls 
sharply  away  and  then  extends  backward  for  a  considerable 
distance  with  a  marked  ventral  inclination.  The  slope  thus 


THE    DIENCEPHALIC    FLOOR  225 

formed  has  its  surface  expression  in  the  post-chiasmatic  eminence 
and  consequently  this  portion  of  the  ventricular  cavity  is  the 
post-chiasmatic  recess  (33).  This  recess  extends  laterally  on 
either  side  of  the  median  line  forming  an  expanded  portion  of 
the  ventricular  cavity.  As  it  is  followed  caudad  the  post-' 
chiasmatic  recess  becomes  constricted  until  it  forms  a  tubular 
canal,  the  infundibular  canal  (12),  which  passes  through  the 
infundibular  stem  to  communicate  with  the  cavity  of  the  in- 
fundibular process,  the  recessus  processi  infundibuli  (40).  The 
post-chiasmatic  recess  and  the  infundibular  canal  were  present 
in  all  the  forms  examined  but  the  distance  to  which  the  canal 
penetrates  the  infundibular  stem  is  variable.  In  the  Felidae 
it  passes  through  the  entire  length  of  the  stem  while  in  all  other 
carnivores  it  extends  a  short  distance  only.  It  is  shortest  in 
ungulates,  anthropoids  and  man,  although  in  these  forms  the 
infundibular  stem  attains  its  greatest  length.  The  recess  of  the 
infundibular  process  is  present,  so  far  as  I  am  at  present  able 
to  state  concerning  mammals,  in  the  Felidae  alone.  In  these 
forms  it  is  in  direct  communication  with  the  third  ventricle 
through  the  infundibular  canal.  In  such  birds  and  reptiles  as 
I  have  studied  it  is  present  as  a  cavity  having  numerous  accessory 
diverticula.  The  general  conformation  of  this  recess  and  the 
infundibular  process  which  contains  it  have  already  been  dis- 
cussed (page  214). 

As  the  dorsal  surface  of  the  post-chiasmatic  eminence  (32) 
ascends  and  reaches  the  plane  of  the  floor,  it  becomes  continuous 
with  an  area  whose  external  expression  is  the  ventral  surface  of 
the  post-infundibular  eminence  (34)-  Entally  this  area  presents 
several  transverse  ridges  which  separate  two  or  three  rather  well 
marked  grooves  extending  transversely  across  the  floor  of  this 
region.  Caudal  to  these  folds  the  floor-plate  becomes  smooth 
and  laterally  a  conspicuous  sinus  or  recess  situated  in  front  of 
the  mammillary  recess  appears  in  all  the  mammals  studied.  Like 
several  of  the  other  structures  already  mentioned,  it  is  a  most 
conspicuous  element  in  the  Felidae. 


226  FREDERICK  TILNEY 

THE  DIENCEPHALIC  FLOOR  OF  THE  ADULT  FOWL  (GALLUS  GALLUS) 

All  of  the  eminences  appearing  in  the  cat  may  be  identified 
in  the  fowl  (fig.  4).  The  supraoptic  crest  (4.1}  is  present  as  a 
ridge  extending  laterad  along  the  dorso-cephalic  border  of  the 
chiasm  toward  the  optic  nerve.  It  marks  the  position  of  the 
supraoptic  recess  (42}  of  the  third  ventricle.  Caudad  to  the 
chiasm  the  diencephalic  floor  forms  a  prominent  post-chiasmatic 
eminence  (32}  which,  as  in  the  case  of  mammals,  is  invested  by 
the  tuberal  portion  of  the  pituitary  gland  and  contains  the 
post-chiasmatic  recess  (38}.  From  the  caudal  extremity  of 
this  eminence  projects  the  infundibular  stem  (11}  terminating 
in  the  infundibular  process  (13}.  In  the  bird  and  the  reptile 
this  process  presents  certain  features  which  distinguish  it  from 
that  of  the  mammal.  It  is  broader  and  each  lateral  extremity 
is  prolonged  to  form  a  slender  lateral  process  (16}  similar  to  the 
lateral  process  of  the  selachian.  Equally  characteristic  are  the 
two  surfaces  of  the  infundibular  process,  i.e.,  the  pituitary  (15} 
and  saccular  (14)  surfaces;  the  former  is  in  contact  with  the 
pituitary  gland;  the  latter  has  no  such  relation  but  presents 
many  irregular  convolutions.  Its  wall  is  thick  and  non- vascular. 
As  the  saccular  surface  of  the  infundibular  process  ascends  and 
approaches  the  general  plane  of  the  ventricular  floor  it  becomes 
evaginated  to  form  a  diverticulum,  the  post-infundibular  emi- 
nence (34}  •  This  structure  seems  to  present  features  in  which 
it  differs  from  the  post-infundibular  eminence  in  the  cat.  It 
appears  to  be  a  constituent  of  the  post-chiasmatic  eminence 
rather  than  being  directly  in  the  floor  of  the  ventricle  as  in  mam- 
mals. In  Botaurus  lentiginosus  (shown  in  fig.  5)  it  more 
closely  resembles  the  mammalian  conditions.  The  avian  third 
ventricle  is  in  general  much  more  narrow  than  in  other  forms. 
Only  in  its  cephalic  extremity  does  it  broaden  out  to  establish 
free  communication  with  the  large  prechiasmatic  and  post- 
chiasmatic  recesses.  In  the  interthalamic  portion  of  the  ven- 
tricle in  the  fowl  the  walls  are  in  close  apposition;  the  extent  of 
the  cavity  in  this  region  is  further  limited  by  the  unusual  size  of 
the  tuberculum  postero-superius  which  projects  cephalad  for  a 


THE    DIENCEPHALIC    FLOOR 


Fig.  4  Mesial  view  of  forebrain  reconstruction  in  adult  fowl  (Gallus  gallus). 
X  50.  The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2, 
chiasmatic  process;  4>  chiasm;  11,  infundibular  stem;  12,  infundibular  canal; 
13,  infundibular  process;  14,  infundibular  process,  saccular  surface;  15,  infun- 
dibular process,  pituitary  surface;  16,  infundibular  process  (lateral  process); 
27,  mammillary  body;  32,  post-chiasmatic  eminence;  33,  post-chiasmatic  recess; 
34,  post-infundibular  eminence;  35,  post-infundibular  recess;  38,  pre-chiasmatic 
recess;  40,  recess  of  the  infundibular  process;  41,  supra-optic  crest;  42,  supra- 
optic  recess. 


considerable  distance  into  the  ventricular  chamber.  The  hypo- 
thalamic  portion  of  the  ventricle  extends  caudad  beneath  the 
ventral  surface  of  the  massive  tuberculum  postero-superius;  for 
this  reason  the  caudal  portion  of  the  post-chiasmatic  eminence 
and  the  post-infundibular  eminence  appear  to  be  appendages 
to  rather  than  constituents  in  the  floor  of  the  third  ventricle  of 
the  fowl.  In  the  bittern  (fig.  5),  on  the  other  hand,  the  post- 
infundibular  eminence  is  in  the  ventricular  floor  while  the 


228 


FREDERICK   TILNEY 


infundibular  process  projects  caudad  as  an  appendage  of  the 
postchiasmatic  eminence.  Both  the  optic  chiasm  (4)  and  the 
chiasmatic  process  (2}  in  the  fowl  are  prominent  and  the  pre- 
chiasmatic  recess  (38)  is  correspondingly  deep.  In  its  general 
features  the  post-chiasmatic  eminence  (32)  is  similar  to  that  in  the 
cat.  It  has  a  long  transverse  axis.  Its  ventral  surface  presents  a 
longitudinal  furrow,  the  long  axis  of  which  is  in  the  median  line. 


42 


15 


Fig.  5  Sagittal  section  of  Botaurus  lentiginosus  in  region  of  interbrain. 
4,  chiasm;  11,  infundibular  stem;  12,  infundibular  canal;  14,  infundibular  proc- 
ess, saccular  surface;  15,  infundibular  process,  pituitary  surface;  26,  mammil- 
lary  recess;  27,  mammillary  body;  34,  post-infundibular  eminence;  35,  post- 
infundibular  recess;  40,  recess  of  infundibular  process;  42,  supra-optic  rpcess. 

This  is  the  median  post-chiasmatic  groove.  Here  the  neural 
tissue  is  relatively  thin.  Extending  laterad  in  both  directions 
from  this  groove  the  surface  of  the  eminence  becomes  convex 
while  the  neural  tissue  rapidly  increases  in  thickness  until  it 
forms  the  prominent  lateral  processes  (21)  of  the  post-chiasmatic 
eminence  which  project  free  of  the  diencephalic  floor.  The 
general  plane  of  the  post-chiasmatic  recess  (33)  is  at  right  angles 
to  the  interthalamic  portion  of  the  ventricle  and  follows  the 
ectal  contour  of  the  post-chiasmatic  eminence.  The  infundib- 
ular canal  (12)  is  short  and  narrow;  it  communicates  directly 
with  the  recess  of  the  infundibular  process  (40).  The  latter 
presents  dorsally  a  number  of  minute  tubular  canals  which 


THE    DIENCEPHALIC    FLOOR  229 

project  into  the  corresponding  diverticula  sacci  vasculosi  (6).' 
These  tubular  canals  open  ventrally  into  a  larger  subdivision 
of  the  recessus  processi  infundibuli  which  is  in  relation  to  the 
pituitary  surface  of  the  infundibular  process,  the  hypophyseal 
recess  (10).  Laterally  the  hypophyseal  recess  may  be  traced 
into  the  two  tapering  lateral  processes  (16}  of  the  infundibular 
process.  The  post-infundibular  recess  (35)  communicates  with 
the  post-chiasmatic  recess  in  a  position  slightly  dorsal  to  the 
infundibular  canal.  In  the  fowl  the  mammillary  bodies  (27) 
are  partly  concealed  by  the  post-infundibular  eminence  but 
appear  as  slight  elevations  in  the  floor  dorso-lateral  to  this  struc- 
ture. Because  of  this  relation  they  seem  to  be  situated  at  some 
little  distance  from  the  median  line  on  eitker  side  having  the 
post-infundibular  eminence  and  its  recess  interposed  between 
them.  They  contain  no  mammillary  recess  in  the  fowl.  In 
the  bittern  the  mammillary  bodies  are  dorso-caudad  to  the  post- 
infundibular  eminence  and  occupy  a  position  much  nearer  to 
the  median  line  than  in  the  fowl.  A  small  mammillary  recess 
extends  for  a  short  distance  into  the  mammillary  body  in  this 
form  (fig.  5). 

THE  DIENCEPHALIC  FLOOR  OF  THE  ADULT  SELACHIAN 
(MUSTELUS  LAEVIS) 

The  mesial  view  of  a  model  reconstructed  to  show  the  left 
half  of  the  interbrain  floor  in  the  adult  Mustelus  laevis  is  re- 
produced in  figure  6.  Of  the  elements  entering  into  the  floor 
the  lamina  terminalis  (20)  is  the  most  cephalic.  The  optic 
chiasm  (4)  is  caudal  to  the  supraoptic  crest  (41)  which  extends 
transversely  across  the  dorso-cephalic  surface  of  the  chiasm, 
and  may  be  followed  for  a  short  distance  out  upon  the  optic 
nerve.  Caudal  to  the  chiasm  is  the  post-chiasmatic  eminence 
which  differs  in  certain  particulars  from  the  corresponding 
region  of  the  mammal  and  sauropsid,  although  it  occupies  the 
same  topographical  position.  The  chief  points  of  difference 
arise  from  the  facts  that  the  three  divisions  of  the  post-chias- 
matic eminence  in  the  dog-fish  are  more  pronounced  than  in  the 


230 


FREDERICK   TILNEY 


^at  and  fowl  and  at  the  same  time  this  region  is  a  relatively 
much  more  expansive  area  than  in  the  forms  already  mentioned. 
Two  of  its  divisions  are  bilaterally  symmetrical  in  the  form  of 
large  ovoid  protuberances  situated  one  on  either  side  of  a  smaller 
median  area.  The  lateral  protuberances  are  the  lobi  inferiores 
;  they  correspond  in  position  to  the  lateral  processes  of  the 


44 


21 


Fig.  6  Mesial  view  of  brain  reconstruction  in  adult  Mustelus  laevis.  X  25. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  3,  cerebellum;  4,  chiasm;  6,  diverticular  sacci  vasculosi;  7,  epiphysis; 
10,  hypophysial  recess;  12,  infundibular  canal;  14,  infundibular  process,  saccular 
surface;  15,  infundibular  process,  pituitary  surface;  20,  lamina  terminalis;  21, 
median  chiasmatic  groove;  24,  mid-brain;  26,  mammillary  recess  (recess  of  pos- 
terior lobe) ;  27,  mammillary  body  (posterior  lobe) ;  82,  post-chiasmatic  eminence 
(inferior  lobe) ;  S3,  post-chiasmatic  recess  (recess  of  inferior  lobe) ;  84,  post- 
infundibular  eminence j  85,  post-infundibular  recess;  39,  paraphysis;  42,  supra- 
optic  recess;  44,  telencephalon;  47,  velum  trans versum. 

post-chiasmatic  eminence  in  birds  and  mammals;  they  are  not 
invested  by  or  in  contact  with  the  pituitary  gland.  The  small 
median  area  corresponds  to  the  median  post-chiasmatic  groove 
(23);  it  is  contiguous  with  a  relatively  long,  tongue-like  process 
of  the  pituitary  gland,  the  developmental  history  of  which  latter 


THE    DIENCEPHALIC    FLOOR  231 

gives  it  all  the  characteristics  of  the  mammalian  and  sauropsidan 
pars  tuberalis.  In  the  bird  and  mammal  the  caudal  extremity 
of  the  post-chiasmatic  eminence  becomes  constricted  to  form  the 
infundibular  stem  (11),  but  in  the  selachian  this  constriction 
is  less  marked  so  that  the  stem  is  wide  and  short.  It  is  difficult 
to  appreciate  it  at  all  except  by  means  of  reconstruction.  At 
its  caudal  extremity  the  stem  becomes  rapidly  expanded  to  form 
the  spacious  infundibular  process  (18).  This  process  presents 
two  characteristic  surfaces,  i.e.,  a  ventral  one  which  is  smooth 
and  in  contact  with  the  pituitary  gland,  the  pituitary  surface: 
and  a  dorsal  convoluted  saccular  surface,  membranous  in 
character  and  highly  vascular.  This  latter  forms  the  saccus 
vasculosus. 

Dorsal  to  the  saccus  vasculosus  is  a  small  protuberance  which 
differs  structurally  from  the  saccus  in  that  it  is  composed  chiefly 
of  neural  tissue.  This  is  the  post-infundibular  eminence  (84), 
dorsal  to  which  is  a  larger  protuberance,  the  posterior  lobe  (27). 
This  structure  forms  a  prominent  eminence  at  the  point  of  junc- 
ture between  the  mid-brain  and  the  interbrain.  Laterally  its 
extremities  project  free  of  the  adjacent  neural  tissue.  These 
lateral  extremities  are  in  connection  with  a  less  protuberant 
median  portion  of  the  lobe.  The  transverse  diameter  of  the 
lobus  posterior  is  about  twice  that  of  the  infundibular  emi- 
nence; both  protuberances  are  symmetrically  disposed  with 
reference  to  the  mid-sagittal  plane.  Cephalad  the  recess  of 
the  posterior  lobe  is  in  direct  communication  with  the  dien- 
cephalic  ventricle;  caudad  it  opens  into  the  post-infundibular 
recess  (35) .  The  recess  of  the  infundibular  process  (40)  is  bounded 
ventrally  by  the  pituitary  surface  of  the  infundibular  process, 
while  caudo-dorsally  it  is  limited  by  the  saccular  surface  forming 
the  saccus  vasculosus.  This  surface  is  thrown  into  a  number 
of  convoluted  folds  thus  producing  the  diverticula  sacci  vasculosi 
(6).  Ventral  to  the  saccus  vasculosus  the  infundibular  recess 
presents  a  marked  subdivision,  the  hypophyseal  recess  (10), 
while  both  the  pituitary  and  saccular  surfaces  are  so  prolonged 
laterad  as  to  form  two  long  tapering  processes,  each  of  which 
contains  a  lateral  extension  of  the  infundibular  recess.  Corre- 


NEUROLOGY,  VOL.  25,   NO. 


232  FREDERICK   TILNEY 

spending  to  the  short,  broad  infundibular  stem,  the  infundib- 
ular canal  (12}  is  not  well  defined,  although  it  may  be  recognized 
as  a  slight  constriction  occurring  dorsad,  at  the  area  of  transition 
between  the  saccus  vasculosus  and  the  post-infundibular  emi- 
nence and  ventrad  in  the  region  in  which  the  pituitary  surface 
of  the  infundibular  process  passes  into  the  median  post-chias- 
matic  groove  (23}.  The  post-chiasmatic  recess  (33)  corresponds 
in  its  subdivisions  to  the  post-chiasmatic  eminence,  there  being 
two  large  lateral  diverticula  extending  into  the  inferior  lobes  in 
connection  with  a  median  canal  which  communicates  cephalad 
with  the  suprachiasmatic  portion  of  the  third  ventricle  and 
caudad  with  the  recess  of  the  infundibular  process.  Cephalad 
to  the  post-chiasmatic  eminence  the  ventricular  floor  is  elevated 
above  the  chiasm  to  form  the  chiasmatic  process  (2}  which 
passes  across  the  floor  as  a  prominent  transverse  ridge.  The 
dorso-cephalic  surface  of  this  ridge  becomes  rapidly  depressed 
as  it  proceeds  cephalad,  and  in  the  mid-sagittal  plane  becomes 
the  caudal  boundary  of  the  prechiasmatic  recess  (38). 

Traced  laterad  this  recess  leads  into  a  small  tubular  canal 
which  extends  for  some  distance  above  the  optic  nerve,  the 
supraoptic  recess  (42).  In  the  selachian  the  lamina  terminalis 
(20)  occupies  a  nearly  horizontal  plane,  extending  with  a  slight 
dorsocephalic  inclination  from  the  prechiasmatic  recess  to  the 
corpus  striatum. 

EMBRYOLOGICAL  ANALYSIS  OF  THE  DIENCEPHALIC  FLOOR  IN  TH  E 
CAT,    CHICK   AND   DOG-FISH 

The  following  analysis  of  the  floor  of  the  interbrain  is  based 
upon  some  recent  work  of  Prof.  H.  von  W.  Schulte  (12),  in  which 
the  writer  collaborated.1  It  is  shown  in  this  study  that  the  fore- 
brain  in  the  cat  consists  of  two  primitive  elements,  the  optic 
vesicles  and  the  mammillary  region.  The  latter  persists  with 
but  little  alteration  until  a  relatively  late  period.  The  primi- 

1  In  connection  with  this  work  it  gives  me  pleasure  to  express  my  indebtedness 
to  Professor  Schulte  for,  although  it. was  my  privilege  to  collaborate  with  him 
in  the  study  of  the  early  development  of  the  brain  in  the  domestic  cat,  the  new 
ontogenetic  interpretation  resulting  from  this  investigation  originated  with  him. 


THE    DIENCEPHALIC    FLOOR  233 

tive  optic  vesicles  early  become  profoundly  remodelled  giving 
rise  to  a  much  reduced  optic  evagination  and  a  pronounced  area 
of  the  neural  wall  which  surrounds  it.  This  area  is  called  the 
ectoptic  zone.  It  presents  itself  as  an  arc  of  three  distinct  seg- 
ments, the  dorsal  segment  giving  rise  to  the  thalamencephalon, 
the  cephalic  segment  to  the  telencephalon  and  the  ventral 
segment  to  the  infundibular  region.  All  of  these  secondary 
derivatives  are  present  in  the  cat  embryo  of  twenty-one  somites 
(see  fig.  xxxviii  in  loc.  cit.). 

Development  of  the  diencephalic  floor  of  tlie  cat 

Cat  embryo  of  4-5  mm.;  twenty-six  somites;  Specimen  No.  495 
(fig.  7).  The  forebrain  of  this  embryo  shows  an  advance  over 
the  conditions  observed  in  the  embryo  of  twenty-one  somites. 
All  of  the  primitive  elements  of  the  prosencephalon  previously 
described  may  be  recognized.  The  optic  vesicles  (29}  are 
further  reduced  in  size,  and  present  a  constriction  at  their  point 
of  attachment  to  the  neural  tube.  Their  external  configuration 
is  still  convex  upon  all  surfaces.  The  ectoptic  zone  shows  its 
characteristic  division  into  thalamencephalon  (43),  telencephalon 
(44)  and  infundibular  region  (17}.  The  regio  mammillaris 
(25}  is  well  marked  and  ectally  separated  from  the  apex  of  the 
infundibular  region  by  a  shallow  transverse  furrow,  the  tubercle 
of  the  floor  (46}. 

The  most  pronounced  changes  are  evident  in  the  regio  infun- 
dibularis,  not  only  in  the  fact  that  this  region  is  enlarged  but  also 
because  it  presents  two  subdivisions,  both  evolved  from  the  apex 
of  this  area.  The  first  of  these  subdivisions  appears  as  a  ventral 
protrusion  which  is  conical  in  shape,  the  infundibular  evagi- 
nation (18}.  Dorsal  and  caudal  to  this  appears  a  large,  shallow 
diverticulum  which,  because. of  its  position,  may  be  designated 
the  post-infundibular  evagination  (36}.  The  remainder  of  the 
infundibular  region  forms  the  greater  part  of  the  floor  of  the 
prosencephalic  ventricle.  Its  general  plane  of  inclination  is 
caudo-ventral,  and  its  thickness  is  uniform  throughout  its  entire 
extent.  Entally  it  presents  on  either  side  a  deep  groove  passing 
along  the  lateral  wall  from  the  now  circular  orifice  of  the  optic 


234 


FREDERICK   TILNEY 


vesicle  to  the  apex  of  the  infundibular  region.  This  is  the 
optico-infundibular  groove  (81).  It  occupies  relatively  the  same 
position  as  the  horizontal  limb  of  the  primitive  optic  sulcus. 
Its  increased  prominence  upon  the  ental  surface  of  the  ven- 
tricular wall  appears  to  have  occasioned  a  corresponding  ridge 
upon  the  ectal  surface,  the  optico-infundibular  ridge. 


18- 


17 


Fig.  7  Mesial  view  of  forebrain  reconstruction  of  a  4.5  mm.  cat  embryo 
(26  somites).  X  150.  The  unshaded  area  shows  the  cut  surfaces  of  the  re- 
construction. 8,  ectoptic  zone  of  Schulte;  17,  infundibular  region;  18,  infundib- 
ular evagination;  24,  mid-brain;  25,  mammillary  evagination;  29,  optic  evagi- 
nation;  31,  optico-infundibular  groove;  36,  post-infundibular  eminence;  4S, 
thalamencephalon;  44,  telencephalon;  46,  tubercle  of  the  floor  of  Schulte. 

The  mammillary  region,  although  increased  in  size  as  com- 
pared with  the  earlier  stages,  manifests  no  other  changes.  Dorsal 
to  it  is  the  tuberculum  postero-superius  (45),  while  ventrad  is 


THE    DIENCEPHALIC    FLOOR 


235 


the  floor  tubercle   of    Schulte   (46),   the  tuberculum    postero- 
inferius. 


44 


17      29 


20 


Fig.  8  Mesial  view  of  forebrain  reconstruction  of  7  mm.  cat  embryo.  X  100. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  13,  infundib- 
ular process;  17,  infundibular  region;  20,  lamina  terminalis;  24,  mid-brain; 
25,  mammillary  region;  29,  optic  evagination;  84,  post-infundibular  eminence; 
37,  post-mammillary  evagination;  43,  thalamencephalon ;  44,  telencephalon;  45, 
tuberculum  postero-superius;  46,  tubercle  of  the  floor  of  Schulte. 

Cat  embryo  of  7  mm.;  Specimen  No.  266  (fig.  8).  Changes  in 
this  embryo  have  occurred  both  in  the  mammillary  and  infun- 
dibular regions.  The  former  now  shows  a  subdivision  into  what 
must  be  considered  the  mammillary  recess  (26),  and  dorsal  to 
this  a  smaller  evagination,  the  post-mammillary  recess  (37). 
Both  of  these  evaginations  affect  the  median  plane,  while  in 
the  mammillary  area  two  lateral  diverticula  have  appeared,  the 
anlages  of  the  mammillary  bodies. 


236  FREDERICK    TILNEY 

The  subsequent  history  of  the  post-mammillary  evagination 
shows  that  this  recess  is  involved  in  the  development  of  the  cor- 
pus interpedunculare.  In  the  infundibular  region,  the  optico- 
infundibular  ridge  is  much  less  prominent  and  the  entire  region 
is  increased  in  size.  Its  notable  characteristics  at  this  stage 
are  a  large  infundibular  process  (IS)  and  a  prominent  post- 
infundibular  eminence  (34)  • 

The  infundibular  evagination  now  contains  an  extension  of  the 
prosencephalic  ventricle,  in  this  respect  differing  from  the  con- 
ditions in  the  4.5  mm.  embryo  in  which  the  infundibular  process 
is  solid  and  as  yet  contains  no  ventricular  extension. 

The  optic  vesicle  (29)  presents  a  distinct  cupping  upon  its 
latero-cephalic  surface  and  is  continuous  with  the  lateral  wall  of 
the  prosencephalon  by  means  of  a  constricted  stem,  the  optic 
peduncle  (30).  Along  the  cephalic  surface  of  this  peduncle 
runs  a  shallow  groove,  which  becomes  expanded  as  it  passes 
out  upon  the  latero-cephalic  surface  of  the  cup.  Entally  a 
groove  connects  the  canal  of  the  two  optic  stems  across  the  floor 
of  the  prosencephalon.  This  is  the  interoptic  groove.  Cephalad 
as  well  as  caudad  to  this  groove  the  floor  is  thin.  In  this  stage, 
therefore,  the  remodelling  of  the  floor  has  resulted  in  the  sub- 
division of  the  mammillary  region,  forming  the  mammillary  and 
post-mammillary  evaginations,  while  laterally  the  anlages  of 
the  mammillary  bodies  have  become  defined.  The  infundibular 
region  likewise  shows  an  advance  in  its  subdivisions,  i.e.,  the 
infundibular  process  (18)  and  the  post-infundibular  eminence 
(34)-  The  rest  of  the  infundibular  regions  still  remains  in  the 
general  plane  of  the  prosencephalic  floor. 

Cat  embryo  of  10  mm.;  Specimen  No.  498  (fig.  9).  The  changes 
in  this  stage  are  more  evident  in  the  telencephalon  and  thala- 
mencephalon  than  in  the  floor-plate.  The  optic  cup  is  now 
more  pronounced  than  in  the  earlier  stage,  the  optic  stem  still 
more  constricted. 

Entally  a  well  marked  interoptic  groove  (19)  is  present,  and 
immediately  caudal  to  this  the  floor-plate  is  thickened  to  form 
the  interoptic  torus.  The  floor  of  the  infundibular  region  from 
the  torus  to  the  infundibular  evagination  is  thin.  The  infundib- 


THE    DIENCEPHALIC    FLOOR 


237 


ular  evagination  shows  a  constriction  at  its  point  of  junction 
with  the  infundibular  region,  in  this  way  demarcating  the 
definitive  infundibular  stem  (11}  and  the  infundibular  process 
(13).  The  ventricular  cavity  extends  through  the  narrow  in- 

24 


44 


17  29  19 

40     12 

Fig.  9  Mesial  view  of  forebrain  reconstruction  of  10  mm.  cat  embryo.  X  100. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  11,  infundib- 
ular stem;  12,  infundibular  canal;  18,  infundibular  process;  17,  infundibular 
region;  19,  inter-optic  groove;  20,  lamina  terminalis;  24,  mid-brain;  25,  mam- 
millary  region;  29,  optic  evagination;  84,  post-infundibular  eminence;  85,  post- 
infundibular  recess;  87,  post-mammillary  evagination;  89,  paraphysis;  40, 
recess  of  the  infundibular  process;  48,  thalamencephalon;  44,  telencephalon; 
45,  tuberculum  postero-superius;  46,  tubercle  of  the  floor  of  Schulte. 

fundibular  process,  thus  giving  rise  to  the  infundibular  canal 
(12)  and  the  infundibular  recess  (40) .  Dorsal  to  the  infundibular 
stem  the  floor  of  the  ventricle  shows  a  large  post-infundibular 
evagination,  which  from  this  time  maintains  the  same  general 


238  FREDERICK   TILNEY 

relation  to  the  infundibular  process  and  for  this  reason  may  be 
designated  the  post-infundibular  eminence  (34)-  The  cavity 
of  this  eminence  which  communicates  with  the  third  ventricle 
forms  the  post-infundibular  recess  (35).  A  slight  ridge  which 
is  the  remnant  of  the  floor  tubercle  (46)  (tuberculum  postero- 
inferius)  separates  the  post-infundibular  recess  from  the  mammil- 
lary  evagination  (26)  which,  as  in  the  7  mm.  embryo,  presents 
two  subdivisions  affecting  the  mid-sagittal  plane,  i.e.,  the 
mammillary  and  post-mammillary  evaginations;  while  laterally 
two  large  diverticula  defining  the  anlages  of  the  mammillary 
body  have  increased  in  prominence  but  still  retain  an  ample 
recess,  the  recessus  mammillaris,  which  is  in  communication 
with  the  third  ventricle.  Dorsal  to  the  post-mammillary 
evagination  is  the  tuberculum  postero-superius  (45)  now  some- 
what increased  in  size. 

Cat  embryo  of  12  mm.;  Specimen  No.  217  (fig.  10).  The 
advance  in  this  embryo  over  that  of  10  mm.  appears  in  the  fact 
that  all  the  definitive  elements  of  the  diencephalic  floor  are  now 
discernible.  The  most  conspicuous  changes  affect  the  region 
of  the  interoptic  groove  and  the  area  caudal  to  it.  Where  this 
optic  groove  formerly  appeared  as  a  furrow  extending  between 
the  orifices  of  the  optic  peduncles,  the  floor  is  still  relatively 
thin;  but  caudal  to  this  groove,  both  entally  and  ectally,  it 
presents  a  pronounced  thickening,  the  ectal  increase  in  size  being 
due  to  the  beginning  formation  of  the  optic  chiasm  (4),  while 
entally  the  thickening  forms  the  chiasmatic  process  (2).  Thus 
the  furrow  in  front  of  the  process  becomes  the  prechiasmatic 
recess  (88).'  Ascending  from  the  latter  the  lamina  terminalis 
(20)  extends  obliquely  cephalo-dorsad  to  join  the  roof-plate. 
Quite  as  notable  as  the  changes  which  have  occurred  in  the 
region  of  the  chiasm  are  those  which  appear  in  the  area  immedi- 
ately caudad  to  it.  Here  the  diencephalic  floor,  without  increas- 
ing in  thickness,  presents  a  ventral  protrusion  which  forms  the 
post-chiasmatic  eminence  (32) .  This  eminence  is  symmetrically 
disposed  with  reference  to  the  mid-sagittal  line.  Caudal  to  this 
is  the  infundibular  stem  (11)  considerably  lengthened  and  ex- 
panding to  form  the  infundibular  process  (13).  Between  the 


THE    DIENCEPHALIC    FLOOR 


239 


42 


Fig.  10  Mesial  view  of  forebrain  reconstruction  of  12  mm.  cat  embryo.  X  100. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  4,  chiasm;  9,  foramen  of  Monro;  11,  infundibular  stem;  12,  infundib- 
ular canal;  IS,  infundibular  process;  20,  lamina  terminalis;  26,  mammillary 
recess;  32,  post-chiasmatic  eminence;  S3,  post-chiasmatic  recess;  34,  post- 
infundibular  eminence;  35,  post-infundibular  recess;  37,  post-mammillary  evagi- 
nation;  38,  pre-chiasmatic  recess;  41 ,  supra-optic  crest;  4&,  supra-optic  recess. 


240  .        FREDERICK   TILNEY 

infundibular  stem  and  the  mammillary  evagination  (26)  is  a 
small  but  distinct  diverticulum  of  the  floor  appearing  as  a  promi- 
nence upon  the  ectal  surface  and  forming  the  post-infundibular 
eminence  (34-}. 

The  mammillary  evagination  is  still  large  and  its  cavity 
spacious.  In  it  may  still  be  recognized  the  subdivisions  already 
described,  i.e.,  the  two  lateral  evaginations  of  the  mammillary 
bodies,  the  median  mammillary  evagination  (26)  and  the  post- 
mammillary  evagination  (37).  All  of  the  eminences  mentioned 
have  their  corresponding  recesses,  i.e.,  the  post-chiasmatic 
recess  (33),  the  post-infundibular  recess  (35),  the  mammillary 
recess  (26)  and  the  post-mamnrillary  recess.  The  ventricle 
extends  through  a  short  and  narrow  infundibular  canal  (12) 
into  a  relatively  large  recess  of  the  infundibular  process. 

The  eye-cup  in  this  stage  is  now  completely  formed.  It  is 
attached  to  the  prosencephalon  by  the  optic  peduncle;  but 
certain  .  changes  have  occurred  in  this  peduncle  which  have 
important  bearings  upon  the  structures  evolved  from  it.  As 
the  peduncle  approaches  the  brain-wall  it  becomes  rapidly 
expanded  in  the  form  of  a  distinct  evagination  of  the  prosenceph- 
alon, into  which  extends  an  expansion  of  the  ventricle.  Ventral 
to  this  evagination  the  optic  peduncle  has  increased  in  thickness, 
due  to  the  appearance  of  fibers  forming  the  optic  nerve  and 
entering  the  optic  chiasm  (4).  In  this  manner  a  diverticulum  of 
the  third  ventricle  comes*  to  overlie  the  lateral  portion  of  the 
optic  chiasm  and  the  proximal  portion  of  the  optic  nerve.  This 
diverticulum  is  the  supraoptic  recess  (42) . 

Cat  embryo  of  15  mm.;  Specimen  No.  505  (fig.  11).  In  this 
embryo  a  foreshortening  has  occurred  in  the  diencephalic  floor. 
This  is  due  principally  to  the  change  in  the  inclination  of  the 
lamina  terminalis  (20)  which  is  now  vertical.  In  consequence 
of  this  alteration  the  prechiasmatic  recess  has  become  the  most 
cephalic  portion  of  the  ventricle.  The  chiasmatic  process  (2) 
and  the  chiasm  have  increased  in  size.  The  post-chiasmatic 
eminence  is  still  further  expanded  and  its  recess  is  larger.  The 
post-infundibular  eminence  (34)  occupies  a  typical  position  be- 
tween the  infundibular  stem  (11)  and  the  mammillary  evagi- 


THE    DIENCEPHALIC    FLOOR 


241 


42 


40    13 

Fig.  11  Mesial  view  of  forebrain  reconstruction  of  15mm.  cat  embryo  X  75. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  4,  chiasm;  11,  infundibular  stem;  12,  infundibular  canal;  13,  infundib- 
ular process;  20,  lamina  tenninalis;  25,  mammillary  region;  32,  post-chiasmatic 
eminence;  34,  post-infundibular  eminence;  35,  post-infundibular  recess;  37,  post- 
mammillary  evagination;  40,  recess  of  infundibular  process;  41,  supra-optic 
crest;  42^  supra-optic  recess. 


nation  (26}.  In  this  latter  evagination  is  possible  to  recognize 
a  median,  a  post-mammillary  and  two  lateral  recesses.  The 
infundibular  stem  has  increased  in  length  so  that  the  infundib- 
ular canal  (12}  is  longer,  while  the  infundibular  process  shows 
a  distinct  thickening  along  its  ventro-cephalic  surface,  thus 


242 


FREDERICK   TILNEY 


giving  evidence  that  the  growth  going  on  in  this  structure  at 
this  stage  is  in  a  cephalic  direction.  The  optic  peduncle  has 
increased  in  size,  due  to  the  addition  of  more  optic  fibers. 

7 


25 


35 


34 


Fig.  12  Mesial  view  of  forebrain  reconstruction  of  30  mm.  cat  embryo.  X  50. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  4,  chiasm;  5,  corpus  interpedunculare ;  7,  epiphysis;  9,  foramen  of 
Monro;  11,  infundibular  stem;  12,  infundibular  canal;  13,  infundibular  process; 
20,  lamina  terminalis;  25,  mammillary  region;  82,  post-chiasmatic  eminence; 
33,  post-chiasmatic  recess;  34,  post-infundibular  eminence;  85,  post-infundib- 
ular recess;  89,  paraphysis;  40,  recess  of  the  infundibular  process;  41,  supra- 
optic  crest;  4%,  supra-optic  recess. 

Cat  embryo  of  30  mm.;  Specimen  No.  585  (fig.  12}.  The  fore- 
shortening of  the  diencephalic  floor  observed  in  the  embryo  of 
15  mm.  is  here  less  pronounced,  although  the  lamina  terminalis 
(20}  retains  its  vertical  position.  The  prechiasmatic  and  supra- 
optic  recesses  are  more  pronounced,  due  to  the  increase  in  size 
of  the  chiasmatic  process  (2} .  The  chiasmatic  fibers  are  present 
in  large  numbers  in  the  ventral  aspect,  while  the  dorsal  extension 


THE    DIENCEPHALIC    FLOOR  243 

of  the  chiasmatic  process  appears  to  be  due  to  the  presence  of 
increasing  numbers  of  commissural  fibers.  The  post-chiasmatic 
eminence  (32)  has  increased  in  size,  both  in  the  median  line  as 
well  as  laterally,  where  it  now  begins  to  present  free  extremities. 
Its  recess  is  deep  and  projects  cephalad  under  the  chiasmatic 
process  in  such  a  way  that  coronal  sections  of  the  brain  in  this 
stage  show  a  distinct  recess  which  extends  forward  beneath  the 
chiasmatic  process.  The  infundibular  stem  and  the  infundibular 
process  have  both  increased  in  size;  the  tendency  of  the  latter 
to  extend  its  growth  cephalad  has  about  ceased,  and  the  entire 
infundibular  process  seems  to  be  on  the  point  of  swinging  dorso- 
caudad  in  its  further  development.  The  general  shape  of  the 
process  at  this  stage  is  oval,  and  the  two  relatively  long  lateral 
processes  appear  on  either  side.  The  infundibular  canal  (12) 
is  relatively  longer  and  the  recess  of  the  infundibular  process 
not  only  more  spacious  but  more  definitely  demarcated  from  the 
canal  than  in  any  earlier  stage. 

Caudad  to  the  infundibular  stem  (11)  is  a  large  post-infundib- 
ular eminence  (34)  containing  a  well  defined  post-infundibular 
recess  (35),  which  latter  is  separated  by  a  transverse  ridge  from 
the  mammillary  recess.  The  neural  wall  bounding  this  recess 
and  thus  forming  the  mammillary  eminence  (27)  has  notably 
increased  in  thickness,  so  that  the  relative  dimensions  of  the 
diencephalic  ventricle  are  being  lessened  by  the  encroachment 
due  to  the  thickening  of  the  brain  floor  in  the  region  of  the 
mammillary  bodies.  This  thickening  particularly  affects  the  two 
lateral  mammillary  diverticula,  while  the  median  mammillary 
and  post-mammillary  evaginations  are  no  longer  discernible. 
Thickening  has  also  occurred  in  the  region  of  the  post-mammil- 
lary evagination,  and  this  area  now  shows  an  ectal  protuberance 
marking  the  site  of  the  corpus  interpedunculare  (5). 

Cat  embryo  of  51  mm.;  Specimen  No.  104  (fig- 13) .  The  changes 
observed  in  this  stage  involve  the  further  development  of  the 
chief  tendencies  observed  in  the  30  mm.  embryo,  i.e.,  the  pro- 
nounced caudal  deflection  of  the  infundibular  process  (18)  which 
appears  as  a  distinct  appendage  to  the  floor  of  the  ventricle,  due 
to  the  increased  length  and  constriction  of  the  infundibular  stem 
(11).  This  process  now  presents  two  surfaces,  one  which  faces 


244 


FREDERICK   TILNEY 


cephalad  and  is  relatively  thick  and  the  other  which  faces  caudad 
and  is  thin.  Both  of  these  surfaces  are  closely  invested  by  the 
pars  infundibularis  of  the  pituitary  gland  and  their  continuation 
laterad  produces  two  long  and  slender  lateral  processes.  The 


27 


Fig.  13  Mesial  view  of  forebrain  reconstruction  of  51  mm.  cat  embryo.  X  50. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  4,  chiasm;  5,  corpus  interpedunculare;  7,  epiphysis;  9,  foramen  of 
Monro;  11,  infundibular  stem;  13,  infundibular  process;  20,  lamina  terminalis; 
27,  mammillary  body;  32,  post-chiasmatic  eminence;  33,  post-chiasmatic  recess; 
35,  post-infundibular  recess;  36,  post-infundibular  evagination;  39,  paraphysis; 
40,  recess  of  the  infundibular  process;  42,  supra-optic  recess. 

brain  floor  in  the  mammillary  region  is  still  further  increased 
in  thickness  and  two  definite  lateral  protuberances,  one  on  either 
side  of  the  median  line,  have  made  their  appearance.  Because 
of  the  increased  thickness  of  the  neural  tissue  which  bounds 
them  the  lateral  diverticula  of  the  mammillary  recess  have 


THE    DIENCEPHALIC    FLOOR 


245 


greatly  decreased  in  size  and  seem  to  be    on  the    point    of 
disappearing. 

The  median  mammillary  and  post-mammillary  evaginations 
are  no  longer  recognizable,  but  a  protuberance  marking  the 
position  of  the  corpus  interpedunculare  (5}  is  still  defined  as  the 


24 


38 


42 


Fig.  14  Mesial  view  of  forebrain  reconstruction  of  70  mm.  cat  embryo.  X  25. 
The  unshaded  area  shows  the  cut  surface  of  the  reconstruction.  2,  chiasmatic 
process;  4>  chiasm;  5,  corpus  interpedunculare;  IS,  infundibular  process;  27, 
mammillary  body;  S2,  post-chiasmatic  eminence;  38,  post-chiasmatic  recess; 
34,  post-infundibular  eminence;  35,  post-infundibular  recess;  38,  pre-chiasmatic 
recess;  40,  recess  of  infundibular  process;  41,  supra-optic  crest;  4®,  supra- 
optic  recess. 


thickened  area  in  that  part  of  the  floor.  The  other  eminences 
and  recesses  defined  in  the  earlier  stages  are  all  present  with  but 
slight  changes. 

Cat  embryo  of  70  mm.;  Specimen  No.  E  70  (fig.  14)  •  In  its 
main  outlines  the  diencephalic  floor  at  this  stage  has  attained 
its  adult  conformation.  It  is  not,  however,  disposed  in  the 


246  FREDERICK   TILNEY 

ultimate  plane  of  the  floor  as  yet,  for  the  region  of  the  post- 
infundibular  and  mammillary  areas  has  assumed  a  more  vertical 
position  than  is  true  of  the  15,  30  and  51  mm.  stages  and  this 
position  will  later  be  so  modified  in  attaining  adult  conditions 
that  the  most  dorsal  element  in  this  region,  the  corpus  inter- 
pedunculare  (5} ,  will  be  rotated  ventrad  through  an  arc  of  nearly 
90°.  All  of  the  eminences  observed  in  the  early  stages  and  their 
corresponding  recesses  are  here  present.  The  infundibular  proc- 
ess (13)  presents  two  well  marked,  lateral  processes,  one  on 
either  side;  the  post-chiasmatic  eminence  (32)  likewise  presents 
two  lateral  processes  which  project  free  beneath  the  supra- 
jacent  lateral  eminences  of  the  tuber  cinereum.  The  wall  of 
the  post-infundibular  eminence  (34)  has  increased  in  thickness 
and  it  bounds  a  spacious  post-infundibular  recess  (35).  Two 
mammillary  bodies  are  now  present,  but  the  mammillary  recess 
(26)  cannot  be  defined  because  of  the  thickening  which  has 
progressed  in  the  development  of  the  mammillary  region.  The 
corpus  interpedunculare  (5)  forms  the  most  dorsal  element  in 
this  portion  of  the  ventricular  floor;  the  prechiasmatic  and 
supraoptic  recesses  (38  and  4%)>  the  latter  extending  out  upon 
the  optic  nerve  for  some  distance,  are  both  present. 

Development  of  the  diencephalic  floor  in  the  chick 

Chick  of  twenty -three  hours;  eight  somites;  Specimen  No.  618 
(Jig.  15).  The  forebrain  at  this  stage  consists  exclusively  of 
the  large  optic  vesicles  similar  in  all  respects  to  the  vesicles  in 
the  cat,  although  their  transverse  diameter  is  greater  and  their 
altitude  less.  Entally  the  horizontal  segment  of  the  optic  sulcus 
is  well  defined,  extending  from  the  deepest  portion  of  the  optic 
evagination  (29)  obliquely  meso-caudad  and  converging  with 
the  corresponding  sulcus  of  the  opposite  side.  The  angle  formed 
by  the  convergence  of  these  two  sulci  is  occupied  by  a  clearly 
defined  prominence,  the  tubercle  of  the  floor  (46).  The  neuro- 
pore  (28)  is  still  open  for  a  considerable  distance  at  the  cephalic 
extremity  of  neural  folds,  but  its  closure  is  more  advanced  than 
in  the  cat  embryo  of  eight  somites. 

Chick  of  forty-nine  hours;  twenty  somites;  Specimen  No.  619 
(fig.  16).  The  advances  in  this  stage  consist  in  the  reduction 


THE    DIENCEPHALIC    FLOOR 
29 


247 


24 


-28 


Fig.  15  Mesial  view  of  forebrain  reconstruction  of  8  somite  chick.  X  150, 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  24,  mid-brain ; 
28,  neuropore;  29,  optic  evagination;  46,  tubercle  of  the  floor  of  Schulte. 


.«     a, 

Fig.  16  Mesial  view  of  forebrain  reconstruction  of  20  somite  chick.  X  loO. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  8,  ectoptic 
zone  of  Schulte;  17,  infundibular  region;  20,  lamina  terminalis;  24,  mid-brain; 
25,  mammillary  region;  29,  optic  evagination;  SI,  optico-infundibular  groove; 
46,  tubercle  of  the  floor  of  Schulte. 


NECHOLOOY,  VOL.  25.  NO.  3 


248 


FREDERICK    TILNEY 


of  the  primitive  optic  vesicles  (29}  and  the  formation  of  the 
ectoptic  zone  (8}  which  now  presents  its  dorsal,  cephalic  and 
ventral  segments.  Further  advance  is  found  in  the  formation 
of  a  definite  mammillary  region  (25).  The  boundary  between 
midbrain  and  forebrain  is  indicated  by  the  tuberculum  postero- 
superius  (45) .  The  ventral  segment  of  the  ectoptic  zone  appears 
as  a  well  defined  infundibular  region  (17)  and  a  wide,  shallow 
groove  extends  from  the  orifice  of  the  optic  evagination  to  the 
apex  of  this  region,  forming  the  optico-infundibular  groove  (31). 
A  slight  transverse  ridge  separates  the  mammillary  and  infundib- 
ular regions  thus  marking  the  position  of  the  tubercle  of  the  floor 

24 


20 


Fig.  17  Mesial  view  of  forebrain  reconstruction  of  6.75  mm.  chick.  X  150. 
17,  infundibular  region;  20,  lamina  terminalis;  24,  mid-brain;  25,  mammillary 
region;  29,  optic  vesicle  or  evagination;  31,  optico  infundibular  groove;  44, 
telencephalon;  45,  tuberculum  postero-superius ;  46,  tubercle  of  the  floor  of 
Schulte. 


THE    DIENCEPHALIC    FLOOR 


249 


(46).     Ectally  the  mammillary  and  infundibular  regions  present 
distinct  eminences. 

Chick  of  108  hours;  6.75  mm.;  Specimen  No.  371  (fig.  17}.  The 
tendency  of  the  infundibular  region  (17)  to  assume  a  more  ventral 
relation  with  reference  to  the  mammillary  region  is  here  more 
pronounced.  Otherwise,  with  the  exception  of  the  greater 


13 


Fig.  18  Mesial  view  of  forebrain  reconstruction  of  chick  of  5  days  and  20 
hours.  X  100.  The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction. 
2,  chiasmatic  process;  4>  chiasm;  7,  epiphysis;  18,  infundibular  process;  20, 
lamina  terminalis;  25,  mammillary  region;  32,  post-chiasmatic  eminence;  3S, 
post-chiasmatic  recess;  36,  post-infundibular  eminence;  38,  pre-optic  recess; 
39,  paraphysis;  41,  supra-optic  crest;  4%,  supra-optic  recess;  44,  telencephalon; 
45,  tuberculum  postero-superius ;  46,  tubercle  of  the  floor  of  Schulte. 


250  FREDERICK   TILNEY 

reduction  of  the  optic  vesicles,  there  are  no  notable  changes  in 
this  stage. 

Chick  of  five  days,  twenty  hours;  Specimen  No.  326  (fig.  18}. 
In  this  stage  a  marked  change  has  occurred  in  the  infundibular 
region  (17}  namely,  the  appearance  of  two  distinct  evaginations 
at  the  apex  of  this  region,  the  more  ventral  of  which  is  involved 
in  the  formation  of  the  infundibular  process  (18),  while  the  more 
dorsal  one  ultimately  forms  the  post-infundibular  eminence 
(34).  These  two  evaginations  of  the  infundibular  region  are 
more  pronounced  in  the  chick  than  in  either  the  cat  or  the  dog- 
fish. The  mammillary  region  (25)  has  increased  in  size,  but 
occupies  a  position  dorsal  to  the  infundibular  region  as  in  the 
immediately  preceding  stage.  The  mammillary  recess  (26) 
is  correspondingly  larger.  Thus  the  floor  of  the  third  ventricle 
now  presents  three  separate  evaginations,  the  infundibular 
evagination  (18),  the  post-infundibular  evagination  (36)  and 
the  mammillary  evagination  (26),  a  condition  corresponding 
in  all  details  to  the  early  history  of  the  development  in  the 
mammillary  and  infundibular  regions  of  the  cat.  At  this  stage 
also  a  large  chiasmatic  process  (2)  has  appeared  thus  demar- 
cating a  prechiasmatic  recess  (38)  and  a  post-chiasmatic  recess 
(33).  This  latter  recess  is  marked  upon  the  exterior  surface  by 
a  prominent  post-chiasmatic  eminence  (32). 

Chick  of  eight  days;  Specimen  No.  315  (fig.  19).  In  this  stage 
all  of  the  definitive  elements  of  the  diencephalic  floor  are  present. 
The  chiasm  (4)  and  the  chiasmatic  process  (2)  have  increased 
in  size  with  the  consequence  that  the  prechiasmatic  and  post- 
chiasmatic  recesses  (38  and  33)  are  more  pronounced.  The 
supraoptic  crest  (41)  and  supraoptic  recess  (42)  are  both  present. 
The  post-chiasmatic  eminence  (32)  has  also  increased  in  promi- 
nence. It  now  shows  a  distinct  longitudinal  furrow  which 
marks  the  inception  of  the  median  post-chiasmatic  groove.  In 
the  region  of  this  groove  the  floor  is  relatively  thin,  while  upon 
either  side  of  it  the  neural  tissue  has  a  considerable  thickness. 
In  the  caudal  portion  of  the  infundibular  region  the  dorsal  and 
ventral  evaginations  are  more  marked  than  in  the  next  earlier 
stage  and  in  them  may  be  distinguished  the  anlages  of  the  in- 
fundibular process  (13)  and  post-infundibular  eminence  (34)- 


THE    DIENCEPHALIC    FLOOR 


251 


The  infundibular  process  manifests  a  tendency  toward  the 
development  of  a  short  infundibular  stem  (11)  while  at  the  sides 
it  gives  the  first  evidence  of  its  lateral  processes  (16).  The 
mammillary  region  presents  two  large  lateral  processes  and  a 
smaller  median  recess  with  surface  markings  corresponding 
with  these  evaginations.  The  mammillary  recess  (26)  as  a 
whole  is  gradually  being  reduced,  due  to  a  thickening  of  its 
lateral  walls. 

24 


Fig.  19  Mesial  view  of  forebrain  reconstruction  of  chick  of  8  days.  X  50. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  3,  cerebellum;  4,  chiasm;  7,  epiphysis;  9,  foramen  of  Monro;  11, 
infundibular  stem;  12,  infundibular  canal;  13,  infundibular  process;  16,  infundib- 
ular process  (lateral  process);  24,  mid-brain;  25,  mammillary  region;  26,  mam- 
millary recess;  S2,  post-chiasmatic  eminence;  86,  post-infundibular  eminence; 
88,  pre-chiasmatic  recess;  41,  supra-optic  crest;  4%,  supra-optic  recess;  44 1 
telencephalon. 

i 

Chick  of  nine  days,  nineteen  hours;  Specimen  No.  919  (fig.  20). 
The  reconstruction  of  this  stage  shows  no  material  change  in  the 
supraoptic  crest  (41),  supraoptic  and  prechiasmatic  recesses 
(42  and  38).  The  chiasmatic  process  (2)  is  less  prominent  than 
in  the  eight-day  chick,  although  the  chiasm  (4)  has  increased 
relatively  in  size.  Caudal  to  the  chiasm  the  post-chiasmatic 
eminence  (32)  has  gained  somewhat  in  prominence.  It  now 


252 


FREDERICK   TILNEY 


shows  clearly  the  median  post-chiasmatic  groove  and  the  two 
lateral  processes,  one  on  either  side  of  this  groove.  More  marked 
is  the  change  in  the  caudal  portion  of  the  infundibular  region 
where  the  infundibular  process  is  now  well  formed,  and  con- 
nected with  the  floor  of  the  diencephalon  by  a  short,  broad  stalk, 
the  infundibular  stem  (11).  The  infundibular  process  itself 

7 


Fig.  20  Mesial  view  of  forebrain  reconstruction  of  chick  of  9  days  and  19 
hours.  X  50.  The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction. 
2,  chiasmatic  process;  8,  cerebellum;  4,  chiasm;  7,  epiphysis;  9,  foramen  of 
Monro;  11,  infundibular  stem;  13,  infundibular  process;  24,  mid-brain;  26, 
mammillary  recess;  27,  mammillary  body;  32,  post-chiasmatic  eminence;  S3, 
post-chiasmatic  recess;  36,  post-infundibular  eminence;  38,  pre-chiasmatic 
eminence;  41,  supra-optic  crest;  4%,  supra-optic  recess;  44,  telencephalon. 

presents  a  median  expanded  portion,  from  which  there  extends 
to  either  side  a  slender  lateral  process.  This  formation  corre- 
sponds closely  t'o  the  conditions  in  the  infundibular  process  of 
Mustelus  at  the  stage  of  50  mm.  The  ventricular  cavity  extends 
through  the  infundibular  stem  and  upon  entering  the  infundib- 
ular process  rapidly  expands  into  a  number  of  branching  diver- 
ticula.  These  diverticula  are  confined  largely  to  the  dorsal 
surface  of  the  infundibular  process  and  extend  from  its  median 


THE    DIENCEPHALIC    FLOOR  253 

portion  into  the  lateral  processes.  At  this  stage,  therefore,  it 
is  possible  to  distinguish  between  a  dorsal  or  saccular  surface 
and  a  ventral  surface,  the  latter  being  in  contact  with  the  anlage 
of  the  pituitary  gland. 

Dorsal  to  the  stem  of  the  infundibular  process  is  a  small  evagi- 
nation  corresponding  in  its  general  relations  to  the  part  already 
described  as  the  anlage  of  the  post-infundibular  eminence  (86). 
Its  growth  has  been  less  pronounced  than  that  of  the  other  parts 
of  the  infundibular  region.  The  cavity  of  the  third  ventricle 
extends  into  it  forming  the  post-infundibular  recess  (85).  In 
the  mammillary  region  (25)  the  tendency  toward  the  reduction 
of  the  ventricular  cavity  already  noted  in  the  chick  of  eight 
days  has  proceeded  still  further.  The  mammillary  region  itself 
forms  a  large  protuberance  dorsal  to  the  post-infundibular  emi- 
nence. The  lateral  median  evaginations  are  still  prominent  on 
the  surface,  but  the  cavities  contained  in  them  have  been  greatly 
reduced  now  forming  small  accessory  recesses  connected  with 
the  third  ventricle.  The  median  mammillary  recess  (26)  is 
still  prominent  sagittally,  although  its  transverse  diameters 
are  much  reduced.  The  mammillary  bodies  are  now  defined 
upon  the  surface. 

Chick  of  fourteen  days,  eighteen  hours;  Specimen  No.  1418 
(fig.  21).  In  this  stage  adult  conditions  have  practically  been 
attained.  The  post-chiasmatic  eminence  (32)  is  less  prominent 
with  the  result  that  the  prechiasmatic  recess  (38)  is  less  well 
defined.  Traced  laterad,  however,  this  recess  may  be  followed 
into  a  small  canal  overlying  the  chiasm  and  proximal  portion 
of  the  optic  nerve,  the  supraoptic  recess  (42).  The  position  of 
this  recess  is  marked  upon  the  surface  by  the  supraoptic  crest 
(41)-  In  the  infundibular  region  the  post-chiasmatic  eminence 
is  well  denned;  its  median  post-chiasmatic  groove  (23)  as 
well  as  its  two  lateral  processes  are  prominent.  The  caudal 
portion  of  the  infundibular  region  shows  but  little  change.  The 
main  portion  of  the  infundibular  process  is  somewhat  larger; 
its  dorsal  convoluted  surface  is  in  even  greater  contrast  to  the 
ventral  pituitary  surface  because  of  its  more  marked  convolution. 
The  third  ventricle  communicates  with  the  infundibular  process 


254 


FREDERICK    TILNEY 


Fig.  21  Mesial  view  of  forebrain  reconstruction  of  14  days  and  18  hours 
chick.  X  25.  1,  aqueduct  of  Sylvius;  2,  chiasmati'c  process;  8,  cerebellum; 
4,  optic  chiasm;  7,  epiphysis;  9,  foramen  of  Monro;  12,  infundibular  canal; 
14,  infundibular  process,  saccular  surface;  15,  infundibular  process,  pituitary 
surface;  24,  mid-brain;  26,  mammillary  recess;  27,  mammillary  body;  32,  post- 
chiasmatic  eminence;  33,  post-chiasmatic  recess;  86,  post-infundibular  emi- 
nence; 38,  pre-chiasmatic  recess;  39,  paraphysis;  41,  supra-optic  groove;  42, 
supra-optic  recess;  44,  telencephalon. 

by  an  extremely  small  canal,  while  the  infundibular  recess 
presents  many  branching  subdivisions  which  extend  to  the 
several  diverticula  of  the  dorsal  convoluted  surface.  The 
post-infundibular  eminence  occupies  its  characteristic  position 
and  has  not  changed  in  size.  It  still  retains  the  post-infundib- 
ular recess  of  the  ventricle. 


THE    DIENCEPHALIC    FLOOR 


255 


In  the  mammillary  region  the  processes  initiated  in  the  stage 
of  eight  days  have  now  proceeded  to  such  a  degree  that  the 
lateral  mammillary  recesses  have  become  obliterated  and  the 
mammillary  bodies  are  now  solid,  containing  no  recess  accessory 
to  the  third  ventricle.  There  is  still  a  slight  remnant  of  the 
median  mammillary  recess  (26],  but  this  also  has  been  consider- 
ably reduced  in  size. 


r-28 


Fig.  22  Mesial  view  of  forebrain  reconstruction  of  3  mm.  Mustelus  laevis. 
X  150.  The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  17, 
infundibular  region;  25,  mammillary  region;  28,  neuropore;  29,  optic  vesicle 
or  evagination;  SI,  optico-infundibular  groove;  45,  tuberculum  postero-superius; 
48,  tubercle  of  the  floor  of  Schulte. 

Development  of  the  diencephalic  floor  in  Mustelus  laevis 

Mustelus  embryo  of  3  mm.;  Specimen  No.  722  (fig.  22}.  The 
embryo  of  this  stage  corresponds  closely  to  the  cat  embryo  of 
eight  somites  and  is  also  similar  to  the  chick  embryo  of  that 
size.  The  neuropore  (28}  is  still  widely  open,  while  the  optic 
vesicle  (29}  forms  the  only  structure  at  the  cephalic  extremity 
of  the  neural  tube.  Ectally  this  vesicle  is  represented  by  a 
marked  lateral  protuberance  of  the  neural  wall  whose  axis  is 
oblique  and  whose  apex  is  directed  caudad.  The  surface  relief 
of  the  brain  at  this  stage  is  shown  in  figure  23.  Dorsal  to  the 


256 


FREDERICK    TILNEY 


apex  of  the  optic  vesicle  is  a  shallow  groove  which  traverses  the 
tube  transversely  and  demarcates  the  vesicle  from  a  prominent 
eminence,  the  mammillary  region  (25).  The  ental  surface  of 
this  model  (fig.  22)  shows  the  optic  vesicle  as  a  shallow  evagi- 
nation  from  the  deepest  portion  of  which  a  sulcus  may  be  traced 
ventrad  and  caudad  to  the  tubercle  of  the  floor  (46).  This  is 
the  optic  sulcus.  The  ectoptic  zone  has  not  yet  made  its  ap- 
pearance. The  mammillary  region  is  well  defined  both  as  an 
ectal  protuberance  and  an  ental  recess.  The  prosencephalon 
thus  presents  the  two  primitive  constituents  which  are  char- 
acteristic of  this  period  in  the  cat  and  chick. 


Fig.  23  Ectal  view  of  forebrain  shown  in  figure  22.  X  150.  The  unshaded 
area  shows  the  cut  surfaces  of  the  reconstruction.  17,  infundibular  region; 
S3,  mammillary  region;  28,  neuropore;  29,  optic  evagination. 

Mustelus  embryo  of  7  mm.;  Specimen  No.  294  (fig-  24)-  In 
this  embryo  the  neuropore  is  still  open,  while  the  advance  in 
development  is  indicated  by  the  marked  reduction  of  the  optic 
vesicle  (29)  and  the  appearance  of  an  ectoptic  zone  (8)  presenting 
the  three  characteristic  segments.  The  dorsal  and  cephalic 
segments  present  respectively  the  anlages  of  the  thalamen- 
cephalon  (43)  and  telencephalon  (44),  while  the  ventral  segment 
now  appears  in  the  form  of  a  definite  infundibular  region  (17). 
The  reduction  of  the  optic  vesicle  has  been  carried  to  such  a 
degree  that  the  optic  cup  and  optic  peduncle  may  both  be  dis- 


THE    DIENCEPHALIC    FLOOR  257 

tinguished,  the  former  manifesting  as  yet  but  slight  cupping, 
the  latter  a  definitely  constricted  stem  attaching  to  the  lateral 
wall  of  the  forebrain.  The  prosencephalic  ventricle  extends 
through  the  optic  peduncle  into  the  spacious  recess  of  the  eye- 
cup.  The  vertical  segment  of  the  optic  sulcus  is  not  present, 

43 


17 

Fig.  24  Mesial  view  of  forebrain  reconstruction  of  7  mm.  Mustelus  laevis. 
X  150.  The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  8, 
ectoptic  zone  of  Schulte;  17,  infundibular  region;  25,  mammillary  region;  28, 
neuropore;  29,  optic  evagination;  31,  optico-infundibular  groove;  43,  thala- 
mencephalon;  44,  telencephalon;  46,  tubercle  of  the  floor  of  Schulte. 

but  the  optico-infundibular  groove  (81)  passes  in  the  position 
of  the  horizontal  segment  of  the  sulcus  from  the  orifice  of  the 
optic  peduncle  (29)  ventro-caudally  to  the  side  of  the  floor 
tubercle  (46).  The  infundibular  and  mammillary  regions  pre- 
sent little  change  in  size.  The  tuberculum  postero-superius 
(45)  is  not  prominent. 


258 


FREDERICK  TILNEY 


Muxtelus  embryo  of  11  mm.;  Specimen  No.  729  (fig.  25}.  The 
midbrain  flexures  are  present  in  this  embryo  and  have  caused 
deflections  of  the  neural  axis  at  the  anterior  and  at  the  posterior 
isthmian  sulci.  The  ectoptic  zone  now  gives  more  definite 
evidence  of  its  ultimate  derivatives,  the  dorsal  segment  showing 

24 


4-1 


Fig.  25  Mesial  view  of  forebrain  reconstruction  of  11  mm.  Mustelus  embryo. 
X  100.  The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  4> 
chiasm;  7,  epiphysis;  .75,  infundibular  evagination;  24,  mid-brain;  25,  mammil- 
lary  region;  29,  optic  evagination;  36,  post-infundibular  evagination;  45,  tubercu- 
lum  postero-superius;  46,  tubercle  of  the  floor  of  Schulte. 

the  characteristic  thalamencephalic  formation,  while  the  cephalic 
segment  presents  all  of  the  primitive  elements  of  the  telencephalon. 
The  ventral  segment  of  the  ectoptic  zone  is  considerably  modified. 
The  infundibular  region  (17}  has  not  only  increased  in  size,  but 
at  its  apex  or  caudal  portion  two  evaginations  have  made  their 
appearance.  Of  these  the  infundibular  evagination  (18}  is 
more  extensive  but  less  protuberant.  The  pituitary  anlage  is 


THE    DIENCEPHALIC    FLOOR  259 

in  contact  with  the  ectal  surface  of  this  evagination,  except  for 
a  small  area  immediately  ventrad  to  the  dorsal  evagination. 
In  these  respects  the  development  of  Mustelus  corresponds  in 
all  details  to  that  of  the  cat  and  of  the  chick.  The  post-infundib- 
ular evagination  (36}  is  the  smaller  of  these  secondary  protuber- 
ances. Upon  its  ventricular  surface  it  is  demarcated  from  the 
mammillary  region  by  a  distinct  elevation,  the  tubercle  of  the 
floor  (46}.  Ventrally  it  becomes  continuous  with  the  ventral 
infundibular  evagination  (18}. 

The  mammillary  region  (25}  has  attained  a  sharper  outline 
without  appreciable  increase  in  size.  It  is  assuming  the  char- 
acteristics which  lead  to  the  recognition  of  it  as  the  posterior  lobe. 
The  optic  vesicle  is  relatively  smaller  than  the  forebrain;  its 
peduncle  (30}  has  become  more  constricted  except  at  its  proximal 
extremity  where  it  presents  a  slight  dilatation  into  which  ex- 
tends a  recess  accessory  to  the  ventricle.  The  optic  peduncle 
still .  retains  its  canal  which  communicates  with  the  residual 
lumen  of  the  optic  cup.  The  optico-infundibular  groove  is  less 
well  denned  than  in  the  earlier  stages. 

Mustelus  embryo  of  20  mm.;  Specimen  No.  730  (figs.  26}.  In 
this  embryo  the  anterior  and  posterior  isthmian  flexures  are 
present  and  well  marked.  All  of  the  elements  described  in  the 
11  mm.  embryo  may  be  recognized  and  are  but  little  changed. 
The  principal  advances  are  seen  in  the  thalamencephalon  and 
telencephalon.  In  the  infundibular  region,  however,  a  process 
of  importance  has  been  initiated  and  has  already  assumed 
considerable  proportions.  During  the  earlier  stages  it  has  been 
in  the  apex  or  caudal  portion  of  this  region  that  notable  changes 
were  observed.  Now  and  for  some  time  to  follow  the  develop- 
ment of  its  cephalic  portion  becomes  more  conspicuous  and 
salient.  Following  the  ventral  border  of  the  optic  peduncle, 
some  optic  fibers  have  already  made  their  way  inward  to  form 
the  chiasm  (4}\  the  floor  of  the  ventricle  at  the  same  time  has 
been  slightly  elevated  above  the  chiasm  as  a  transverse  ridge 
passing  between  the  orifices  of  the  optic  peduncles.  This  ridge, 
the  chiasmatic  process  (2} ,  separates  the  prechiasmatic  and  post- 
chiasmatic  recesses.  Of  these  the  latter  is  the  more  prominent 


260 


FREDERICK   TILNEY 


44 


Fig.  26  Mesial  view  of  forebrain  reconstruction  of  20  mm.  Mustelus.  X  75. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  3,  cerebellum;  4,  chiasm;  7,  epiphysis;  18,  infundibular  evagination; 
24,  mid-brain;  25,  mammillary  region;  32,  post-chiasmatic  eminence;  38,  post- 
chiasmatic  recess;  36,  post-infundibular  eminence;  41  >  supra-optic  crest;  4%, 
supra-optic  recess;  44,  telencephalon;  46,  tubercle  of  the  floor  of  Schulte;  47, 
velum  trans versum. 


and  has  expression  upon  the  surface  in  a  large  protuberance 
caudal  to  the  chiasm,  the  post-chiasmatic  eminence.  This 
eminence  already  shows  a  demarcation  into  a  shallow  post- 
chiasmatic  groove  in  which  the  ventricular  floor  is  thin  and  two 
large,  thick-walled  protuberances,  the  lateral  processes  of  the 
post-chiasmatic  eminence  or  inferior  lobes.  The  cavity  of  the 


THE    DIENCEPHALIC    FLOOR 


261 


ventricle  extends  from  the  narrow  median  portion  of  the  post- 
chiasmatic  eminence  into  the  two  lateral  processes,  thus  forming 
the  recessus  lobi  inferioris.  From  the  dorsal  region  of  the 
pituitary  anlage  two  sprouts,  each  as  yet  independent  of  the 
other,  are  growing  forward  along  the  ventral  surface  of  the  median 
post-chiasmatic  groove. 

Mustelus  of  50  and  70  mm.;  Specimens  Nos.  725  and  735 (figs.  27 
and  28} .    The  development  in  these  stages  has  carried  the  differ- 


XL 


40 


13 


Fig.  27  Mesial  view  of  forebrain  reconstruction  of  50  mm.  Mustelus.  X  50. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  4,  chiasm;  7,  epiphysis;  13,  infundibular  process;  24,  mid-brain;  25, 
mammillary  region;  32,  post-chiasmatic  eminence  (lobus  inferior);  33,  post-chias- 
matic recess  (recess  of  inferior  lobe);  36,  post-infundibular  evagination;  39, 
paraphysis;  41,  supra-optic  crest;  4®,  supra-optic  recess;  44,  telencephalon;  47> 
velum  transversum. 


262 


FREDERICK   TILNEY 


entiation  of  the  post-chiasmatic  eminence  still  further  and 
makes  it  possible  to  recognize  more  clearly  in  the  two  lateral 
processes  of  this  eminence  the  anlages  of  the  lobi  inferiores.  The 
median  post-chiasmatic  groove  remains  as  a  narrow  strip  of  the 
brain  floor  in  which  the  tissue  is  relatively  thin,  while  the  inferior 
lobes  are  becoming  massive,  thick-walled  bodies.  The  recess 

44 


24 


41 


42 


18          ^ f T          2 

3332 

Fig.  28  Mesial  view  of  forebrain  reconstruction  of  70  mm.  Mustelus.  X  50. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  4,  chiasm;  7,  epiphysis;  18,  infundibular  evagination;  26,  mammillary 
recess;  27,  mammillary  body  (posterior  lobe);  32,  post-chiasmatic  eminence 
(inferior  lobe);  S3,  post-chiasmatic  recess  (recess  of  inferior  lobe);  35,  post- 
infundibular  recess;  36,  post-infundibular  eminence;  39,  paraphysis;  40,  recess 
of  infundibular  process;  41,  supra-optic  crest;  42,  supra-optic  recess;  44, 
telencephalon. 

of  the  ventricle  in  relation  with  the  region  of  the  median  groove 
extends  laterad  upon  both  sides  into  the  inferior  lobes.  The 
chiasmatic  process  (2)  has  increased  in  size  and  consequently 
projects  further  into  the  ventricle,  thus  accentuating  the  bounda- 
ries of  the  prechiasmatic  and  post-chiasmatic  recesses.  The 
former  recess  may  be  traced  laterad  into  a  small  canal  which 
overlies  the  optic  chiasm  and  the  proximal  portion  of  the  optic 


THE    DIENCEPHALIC    FLOOR  263 

nerve,  the  supraoptic  recess  (42}.  The  caudal  portion  of  the 
infundibular  region  presents  no  change  of  moment.  The  two 
sprouts  of  the  pituitary  anlage  which  extend  along  the  ventral 
surface  of  the  median  post-chiasmatic  groove  are  still  independ- 
ent of  each  other  except  for  a  small  area  near  their  point  of  origin 
where  they  seem  to  have  undergone  fusion  across  the  median 
line.  The  mammillary  region  (25}  has  extended  to  some  degree 
laterally  so  that  now  there  are  added  to  the  original  median 
evagination  of  this  region  two  slightly  projecting  lateral  proc- 
esses into  which  the  cavity  of  the  ventricle  extends.  In  the 
70  mm.  Mustelus  the  inferior  lobes  are  more  prominent  and  the 
pituitary  sprouts  extending  along  the  ventral  surface  of  the  me- 
dian post-chiasmatic  groove  have  now  united  throughout  their 
entire  length,  thus  forming  a  tongue-like  projection  of  the  pitui- 
tary gland.  The  mammillary  region  in  this  stage  has  attained 
all  of  the  characteristics  of  the  posterior  lobe  (27},  presenting  a 
median  portion  and  two  lateral  processes  which  project  free. 
In  the  caudal  area  of  the  infundibular  region  the  only  notable 
change  is  in  the  apparent  expansion  of  the  ventral  evagination 
in  that  portion  immediately  ventral  to  the  dorsal  evagination. 
This  is  an  area  which  is  not  in  contact  at  any  point  with  the 
pituitary  gland. 

Mustelus  of  100  and  300  mm.;  Specimens  Nos.  726  and  694 
(figs.  29  and  30}.  In  both  of  these  late  stages  the  angle  in  the 
neural  axis  at  the  posterior  isthmian  sulcus  has  disappeared. 
The  optic  chiasm  (4)  and  chiasmatic  process  are  larger,  thus 
accentuating  the  prechiasmatic  (88},  and  the  post-chiasmatic 
(33}  recesses.  The  supraoptic  recess  (42}  extends  as  a  small 
canal  along  the  cephalic  surface  of  the  chiasm  and  the  proximal 
portion  of  the  optic  nerve.  In  the  post-chiasmatic  eminence 
(32}  the  inferior  lobes  and  the  median  post-chiasmatic  groove 
may  be  definitely  recognized.  In  the  groove  rests  the  tongue- 
like  extension  of  the  pituitary  gland.  The  posterior  lobe  (27} 
has  increased  in  size,  especially  in  its  lateral  processes,  each  of 
which  contains  an  accessory  recess  of  the  ventricle.  More 
prominent  changes,  however,  have  appeared  in  the  caudal 
portion  of  the  infundibular  region  and  particularly  in  the  ventral 

THE   JOURNAL   OF  COMPARATIVE    NEUROLOGY,   VOL.   25,   NO.    3 


264 


FREDERICK   TILNEY 


evagination  (IS).  Here  growth  has  been  more  pronounced  than 
in  any  other  of  the  neighboring  parts.  This  growth  has  affected 
the  area  immediately  ventrad  to  the  dorsal  infundibular  evagi- 
nation which  in  this  stage  of  70  mm.  had  already  shown  signs 
of  expansion.  The  result  of  the  growth  is  the  formation  of  an 
infundibular  process  (18)  which  consists  of  a  smooth,  membran- 
ous, but  as  yet  non-vascular,  dorsal  surface  not  in  contact  with 

24 
36       27  3 


Fig.  29  Mesial  view  of  brain  reconstruction  of  100  mm.  Mustelus.  X  25. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  3,  cerebellum;  4, ^chiasm;  7,  epiphysis;  13,  infundibular  process;  14, 
infundibular  process,  saccular  surface;  15,  infundibular  process,  pituitary  sur- 
face; 24,  mid-brain;  27,  mammillary  body  (posterior  lobe);  32,  post-chiasmatic 
eminence  (lobus  inferior) ;  33,  post-chiasmatic  recess  (recess  of  inferior  lobe) ; 
S6,  post-infundibular  evagination;  40,  recess  of  infundibular  process;  41>  supra- 
optic  crest;  42,  supra-optic  recess;  44,  telencephalon;  47,  velum  transversum. 


the  pituitary  gland,  the  saccular  surface  (14)  and  a  thicker  sur- 
face in  contact  with  the  gland,  the  pituitary  surface  (15).  In 
the  100  mm.  Mustelus  the  saccular  surface  presents  no  sign  of 
convolution  or  blood  vessels;  in  the  300  mm.  Mustelus  the  ten- 
dency toward  the(  production  of  complicated  diverticula  in  this 
surface  is  evident,  as  well  as  a  rapidly  advancing  vascularization. 
These  changes  mark  the  inception  not  only  of  the  diverticula 
sacci  vasculosi,  but  also  of  the  rich  blood  supply  which  gives  the 
saccus  vasculosus  its  name.  In  addition  to  the  diverticula 
sacci  vasculosi  there  is  a  portion  of  the  recess  of  the  infundibular 


THE    DIENCEPHALIC    FLOOR  265 

process  which,  because  of  its  close  relation  to  the  pituitary  gland, 
is  called  the  recessus  hypophyseus  (10}.  The  pituitary  and 
saccular  surfaces  of  the  infundibular  process  are  continued  laterad 
for  a  considerable  distance  forming  two  tapering  extensions,  the 
lateral  processes  of  the  processus  infundibuli.  During  this 
development  the  dorsal  infundibular  evagination  (36)  manifests 
but  little  change;  when,  however,  the  infundibular  process  is 
formed  the  dorsal  evagination,  because  of  its  position,  becomes 
the  post-infundibular  eminence. 

3 


44 


Fig.  30  Mesial  view  of  brain  reconstruction  of  300  mm.  Mustelus.  X  25. 
The  unshaded  area  shows  the  cut  surfaces  of  the  reconstruction.  2,  chiasmatic 
process;  3,  cerebellum;  4,  chiasm;  7,  epiphysis;  10,  hypophyseal  recess;  IS, 
infundibular  process;  14,  infundibular  process,  saccular  surface;  15,  infundibular 
process,  pituitary  surface;  24,  mid-brain;  27,  mammillary  body  (posterior  lobe); 
32,  post-chiasmatic  eminence  (inferior  lobe);  83,  post-chiasmatic  recess  (recess 
of  inferior  lobe);  36,  post-infundibular  evagination;  39,  paraphysis;  40,  recess 
of  the  infundibular  process;  41,  supra-optic  crest;  42,  supra-optic  recess;  44, 
telencephalon;  47,  velum  transversum. 

The  ontogenesis  of  the  diencephalic  floor  jn  Mustelus,  as  in 
the  cat  and  chick,  may  thus  be  traced  from  two  primitive  fore- 
brain  elements,  the  optic  vesicle  and  the  mammillary  region. 
From  the  optic  vesicle  is  derived  the  ventral  segment  of  the  ectop- 
tic  zone  which  gives  rise  to  the  infundibular  region.  From  the 
optic  vesicle  the  following  structures  are  derived,  i.e.,  the  retina, 


2()()  FREDERICK   TILNEY 

optic  nerve  and  chiasm,  chiasmatic  process,  prechiasmatic  and 
supraoptic  recesses  of  the  third  ventricle.  The  derivatives  of 
the  ventral  evagination  of  the  infundibular  region  in  Mustelus 
are  the  inferior  lobes,  median  post-chiasmatic  groove  and  the 
infundibular  process  (including  the  pituitary  surface,  the  saccus 
vasculosus  and  the  lateral  processes  of  the  saccus).  From  the 
dorsal  evagination  arises  the  post-infundibular  eminence.  The 
mammillary  region  becomes  the  posterior  lobe. 

DISCUSSION 

A  review  of  the  literature  covering  the  diencephalon  reveals 
certain  difficulties  imposed  upon  this  region  by  terminology. 
These  difficulties  are  especially  pronounced  in  the  basal  region 
of  the  interbrain,  which  His  (10)  has  called  the  hypothalamus. 
The  advantage  of  this  term  in  the  adult  human  brain,  as  well  as 
in  other  mammalian  forms,  is  obvious;  but  it  loses  its  precision 
when  applied  to  many  of  the  lower  vertebrates.  There  is  much 
to  recommend  the  term  hypencephalon  (Unterhirn)  suggested 
by  von  Kuppfer  (7).  This,  however,  refers  to  a  somewhat 
arbitrary  subdivision  of  the  diencephalon,  and  von  Kuppfer 
himself  is  not  always,  consistent  in  its  application,  since  in  some 
instances  he  uses  it  as  the  equivalent  of  the  hypothalamus,  while 
in  others  he  restricts  it  to  the  basal  area  of  the  interbrain,  ex- 
clusive of  the  infundibulum.  The  latter  element,  'infundibulum,' 
more  perhaps  than  any  other  part  in  this  region,  has  been  a 
source  of  confusion.  Johnston's  (11)  objections  to  the  use  of 
the  term  'infundibulum'  seem  well  founded.  As  he  points  out, 
the  application  of  this  term  to  a  relatively  extensive  embryologi- 
cal  area  and  also  to  a  much  more  restricted  portion  of  the  adult 
brain  is  almost  certain  to  be  misleading.  Usage,  however,  has 
given  it  a  permanency  in  the  literature  and  in  addition  to  this 
the  phylogenetic  significance  attached  to  the  part  by  some 
investigators  cannot  be  overlooked.  As  already  stated,  Ayers 
(2)  asserts  that  the  hypophysis  arose  as  an  organ  of  taste  and 
the  infundibulum  was  its  nerve.  Boeke  (19)  has  found  in  Murae- 
noids  from  the  third  day  of  embryonic  life  until  the  critical 


THE    DIENCEPHALIC    FLOOR  267 

period  a  sharply  defined  area  in  the  ventral  infundibular  wall 
clearly  differentiated  to  form  not  a  gland  as  some  maintain  but 
a  sense  organ  of  unknown  significance  which  is  functional  in 
the  early  larval  stages.  Other  pelagic  teleosts  present  a  similar 
structure,  although  not  as  clearly  marked  as  in  the  Muraenoids. 

The  ectoptic  zone.  It  has  seemed  to  the  writer  that  some  of  the 
discrepancies  above  mentioned  may  arise  from  the  fact  that  the 
'infundibulum'  in  the  embryological  sense  is  not  one  of  the  primi- 
tive areas  of  the  diencephalon.  It  is,  as  already  shown  in  the 
domestic  cat  by  Schulte  and  Tilney  (12),  a  secondary  derivative 
of  the  primitive  optic  vesicle;  interpreted  in  this  light  its  signifi- 
cance seems  to  become  more  clear.  In  the  domestic  cat,  the 
chick  and  the  dog-fish  two  primitive  areas  may  be  recognized 
in  the  developing  forebrain.  For  a  considerable  period  before 
the  neural  folds  meet  in  the  region  of  the  prosencephalon  the 
optic  vesicles  are  the  only  elements  present.  They  appear  at  the 
cephalic  extremity  of  the  neural  folds  as  prominent  diverticula, 
one  upon  either  side.  Almost  immediately  after  the  formation 
of  the  vesicles  and  before  the  neural  folds  have  met,  there  appears 
a  recess  in  the  floor  of  the  prosencephalon  directly  caudal  of  the 
apex  of  the  optic  evagination.  This  recess  presents  a  correspond- 
ing ectal  protuberance  which  forms  the  mammillary  region. 
The  optic  vesicle  and  mammillary  region  are  consequently  the 
primitive  derivatives  of  the  cephalic  extremity  of  the  neural 
tube.  These  observations  are  true  of  the  cat,  chick  and  dog- 
fish. In  the  subsequent  evolution  of  the  prosencephalon  the 
optic  vesicles  play  the  more  important  role  of  these  two  primitive 
derivatives.  For  a  period  prior  to  and  for  some  time  after 
closure  of  the  neuropore  this  vesicle  undergoes  a  profound 
remodelling,  as  a  result  of  which  the  optic  evaginations  become 
reduced  in  size  and  an  ectoptic  zone  appears  in  the  form  of  an 
arc  about  the  vesicle.  This  arc  presents  a  dorsal,  a  cephalic 
and  a  ventral  segment,  the  latter  being  the  last  to  make  its  ap- 
pearance. When,  however,  it  has  appeared  it  constitutes  a 
well  defined  area,  the  infundibular  region. 

It  has  already  been  shown  (p.  233)  that  the  telencephalon 
arises  from  the  cephalic  segment  of  the  ectoptic  zone,  while  the 


268  FREDERICK   TILNEY 

thalamencephalon  takes  origin  in  the  dorsal  segment.  From 
these  facts  it  seems  fair  to  assume  that  the  third  element  of  this 
zone,  namely  the  ventral  segment,  possesses  similar  possibilities 
of  development  and  the  three  segments  of  the  ectoptic  zone  may 
therefore  be  regarded  as  dynamically  coordinate.  The  attempt 
to  substantiate  this  assumption  is  given  in  discussing  the  de- 
rivatives of  the  infundibular  region. 

The  optic  vesicles.  In  the  period  of  development  shortly  after 
the  closure  of  the  neuropore  the  floor  of  the  forebrain  consists 
of  the  lamina  terminalis,  the  optic  evaginations  and  the  infundib- 
ular and  mammillary  regions.  In  sauropsid  and  mammalian 
forms  the  lamina  terminalis  assumes  a  vertical  position  in  the 
later  stages  of  development;  in  the  ichthyopsid,  on  the  contrary, 
it  remains  horizontal  and  is  thus  an  element  in  the  floor  of  the 
ventricle  cephalad  to  the  optic  chiasm.  Entally  the  evaginations 
of  the  primitive  optic  vesicles  are  indicated  by  the  optic  sulcus 
which  does  not  in  any  way  correspond  to  the  preoptic  and  post- 
optic  grooves  of  authors.  This  sulcus  is  bilateral.  It  does  not 
cross  the  median  line  as  the  above  mentioned  grooves  are  shown 
to  do,  and  in  its  early  appearance  it  presents  itself  as  an  arcuate 
fissure  consisting  of  a  vertical  and  horizontal  segment,  the  latter 
extending  caudad  to  the  tubercle  of  the  floor,  while  the  convexity 
of  the  entire  arc  is  directed  cephalad.  Subsequently,  when  the 
primitive  optic  vesicle  has  become  reduced  by  the  remodelling 
which  results  in  the  formation  of  the  ectoptic  zone,  the  vertical 
segment  of  the  optic  sulcus  disappears  but  the  position  of  its 
horizontal  segment  is  occupied  by  the  optico-infundibular  groove. 
At  this  stage  the  optic  vesicle  has  so  changed  its  external  con- 
formation as  to  have  the  appearance  of  a  pedunculated  divertic- 
ulum  of  the  forebrain  to  which  latter  it  is  attached  by  a  con- 
stricted, hollow  stalk,  the  optic  peduncle.  In  relatively  late 
stages  after  the  appearance  of  the  chiasmatic  process  a  transverse 
groove  connects  the  orifices  of  the  optic  peduncles.  A  groove 
is  also  formed  caudad  to  the  chiasmatic  process.  These  undoubt- 
edly correspond  to  the  post-optic  and  preoptic  grooves  already 
mentioned,  but  it  will  be  noticed  that  their  appearance  develop- 
mentally  is  relatively  late.  The  proximal  portion  of  the  optic 


THE    DIENCEPHALIC    FLOOR  269 

peduncle  ultimately  becomes  expanded  and  contains  a  large 
recess  accessory  to  the  third  ventricle  which  overlies  the  optic 
chiasm  and  proximal  portion  of  the  optic  nerve.  The  remainder 
of  the  optic  diverticulum  rapidly  assumes  the  characteristic 
form  of  the  eye-cup,  while  the  cavity  between  its  ental  and  ectal 
layers  communicates  with  the  third  ventricle  by  means  of  the 
optic  peduncle,  retaining  this  communication  until  a  late  period 
of  embryonic  life.  Ultimately  the  lumen  of  the  distal  portion 
of  the  peduncle  becomes  obliterated.  In  the  adults  of  all  the 
forms  examined  a  marked  prechiasmatic  recess  of  the  ventricle 
is  present.  It  is  continued  laterad  along  the  dorso-cephalic 
margin  of  the  chiasm  and  for  some  distance  above  the  optic 
nerve  as  the  supraoptic  recess.  The  position  of  these  recesses 
is  indicated  upon  the  surface  by  a  ridge  which  traverses  the  chiasm 
and  proximal  portion  of  the  nerve,  the  supraoptic  crest.  This 
likewise  is  constant  in  all  the  forms  examined.  Osborn  (13), 
Herrick  (14)  and  Kingsbury  (15)  have  shown  that  the  rudi- 
mentary condition  of  the  eye  in  Necturus  is  accompanied  by  a 
similar  condition  of  the  optic  nerve  which  retains  the  primitive 
lumen  of  the  optic  vesicle  and  is  hollow  for  a  considerable  dis- 
tance peripherad.  This  fact  and  the  embryological  evidence- 
make  clear  the  homologies  of  the  supraoptic  crest,  chiasmatic 
process,  prechiasmatic  and  supraoptic  recesses  in  the  adult. 

The  infundibular  region.  The  infundibular  region  presents 
greater  difficulties  for  analysis.  These  doubtless  are  due  to  the 
fact  that  the  ventral  segment  of  the  ectoptic  zone,  like  the 
cephalic  and  dorsal  segments,  is  capable  of  marked  adaptive 
variations.  As  seen  in  the  elasmobranch,  for  example,  the  main 
feature  of  this  region  is  the  so-called  hypoarium  described  and 
first  so  named  by  Sanders  (16).  The  hypoarium  consists  of 
two  large  symmetrical  eminences  situated  one  on  either  side  of 
the  mid-sagittal  line  immediately  caudad  to  the  optic  chiasm  and 
containing  an  extension  of  the  third  ventricle.  In  their  general 
conformation  they  are  not  unlike  the  optic  lobes  of  the  mid- 
brain;  they  are  usually  termed  the  lobi  inferiores.  In  many 
teleosts  these  lobes  are  still  further  subdivided  forming  on  either 
side  a  large  lateral  lobe  and  a  small  inferior  lobe,  both  of  which 


270  FREDERICK   TILNEY 

contain  extensions  of  the  third  ventricle.  In  amphibia  the 
inferior  lobes  are  prominent  elements  derived  from  the  infundib- 
ular region,  while  in  sauropsids  and  mammals  the  tendency  to 
the  formation  of  the  extensive  hypoarium  appears  to  have  ceased 
and  no  corresponding  structures  are  to  be  found  in  the  position 
of  the  inferior  lobes.  Fritsch  (17)  suggested  the  homology  of 
these  structures  with  the  corpora  mammillaria,  but  Herrick  (14) 
and  others  have  shown  conclusively  that  this  suggestion  is  not 
well  founded.  The  significance  of  the  inferior  lobes  becomes 
more  apparent  in  the  light  of  their  embryological  history.  As 
already  stated,  the  infundibular  region  in  embryos  of  the  dog- 
fish, chick  and  cat  is  a  secondary  derivative  from  the  primitive 
optic  vesicles.  After  this  region  has  made  its  appearance  the 
portion  immediately  caudad  to  the  optic  chiasm  undergoes 
certain  changes  which  in  the  selachian  terminate  in  the  formation 
of  the  inferior  lobes.  The  development  of  these  structures  as 
definitive  elements  in  the  floor  of  the  diencephalon  begins  at  a 
relatively  late  period  and  is  characterized  by  the  growth  of  a 
diverticulum  immediately  behind  the  chiasm  in  such  a  way  that 
two  symmetrical  evaginations  are  formed,  each  containing  an 
extension  of  the  third  ventricle.  These  bilateral  evaginations 
at  first  have  thin  walls  but  they  grow  rapidly,  the  walls  becoming 
thicker  and  the  cavity  contained  within  them  being  correspond- 
ingly reduced  in  size.  In  the  bird  the  same  tendency  to  the 
formation  of  a  large  diverticulum  immediately  caudad  to  the 
chiasm  is  observed.  This  diverticulum  in  the  relatively  late 
stages  tends  to  become  divided  into  two  symmetrical  evaginations 
and  at  this  stage  resembles  in  all  details  the  early  formation  of 
the  inferior  lobes  in  the  selachian.  Thereafter,  however,  the 
impetus  toward  the  formation  of  the  typical  ichthyopsid  hypoar- 
ium appears  to  cease.  The  evaginations  remain  comparatively 
thin-walled  and  finally  become  a  fairly  prominent  post-chias- 
matic  eminence.  A  similar  course  of  events  is  observed  in  the 
mammal  as  illustrated  by  the  development  of  the  domestic  cat. 
Here  the  infundibular  region  caudad  to  the  chiasm  at  first  forms 
a  diverticulum  which  later  becomes  subdivided  sagittally  in  such 
a  way  as  to  present  two  bilateral  evaginations.  The  conditions 


THE    DIENCEPHALIC    FLOOR  271 

observed  in  this  region  in  the  cat  embryo  of  25  mm.  length  al- 
most exactly  duplicate  those  in  the  brain  of  the  20  mm.  Mustelus, 
but  as  in  the  case  of  the  chick,  the  tendency  of  this  region  to 
give  rise  to  large  inferior  lobes  diminishes  as  growth  progresses 
and  in  the  fetal  stages  the  only  evidence  of  this  tendency  is  to 
be  found  in  the  large  post-chiasmatic  eminence  and  the  post- 
chiasmatic  recess.  Significant  in  this  connection  also  is  the 
relation  which  the  pituitary  gland  bears  to  this  part  of  the 
infundibular  region.  As  the  writer  (8)  has  already  shown,  the 
post-chiasmatic  eminence  in  the  cat  and  chick  comes  to  be  in- 
vested by  a  secondary  outgrowth  from  Rathke's  pocket  which 
ultimately  forms  the  pars  tuberalis  of  the  gland.  In  the  selachian 
no  such  investment  of  the  inferior  lobes  takes  place,  but  the 
tuberal  portion  of  the  gland  by  a  process  of  development  similar 
to  that  in  the  bird  and  mammal  grows  forward  and  occupies 
a  juxta-neural  position  in  contact  with  the  small  portion  of  the 
infundibular  region  which  forms  the  median  post-chiasmatic 
groove. 

The  caudal  portion  of  the  infundibular  region  including  the 
apex  is  involved  in  the  formation  of  the  neural  portion  of  the 
hypophysis.  In  its  inception  the  developmental  process  of  this 
part  in  all  the  forms  studied  presents  a  marked  similarity.  The 
area  about  the  apex  of  the  infundibular  region  undergoes  a  sub- 
division so  that  two  evaginations  are  formed,  one  dorsal  and  the 
other  ventral.  In  the  cat  this  subdivision  gives  the  dorsal 
evagination  the  greater  size  from  its  beginning,  while  in  the  bird 
and  the  dog-fish  the  ventral  evagination  is  the  larger.  In  all 
instances  the  ventral  evagination  proceeds  to  the  formation  of 
the  neural  portion  of  the  hypophysis,  the  infundibular  process. 
In  the  salachian  this  process  is  but  little  pedunculated.  A  slight 
constriction  does,  however,  occur  and  justifies  the  term  infundib- 
ular process  for  the  structure  which  is  connected  with  the  floor 
of  the  interbrain  by  a  short  infundibular  stem.  The  characteris- 
tic features  of  this  infundibular  process  are  the  development  of 
two  morphologically  different  surfaces  one  which  is  ventral, 
thin  and  non-convoluted  coming  in  contact  with  the  pituitary 
gland,  the  other  which  is  thin-walled  and  .dorsal  in  position,  but 


272  FREDERICK-  TILNEY 

having  no  contact  with  this  gland.  The  ventral  surface  because 
of  its  relations  may  be  called  the  pituitary  surface,  the  dorsal 
one  because  of  its  participation  in  the  formation  of  the  saccus 
vasculosus,  the  saccular  surface.  These  two  surfaces  are  pres- 
ent in  the  bird;  but  in  the  cat,  although  the  dorsal  surface  of 
the  infundibular  process  is  much  thinner  than  its  ventral  surface, 
there  is  no  other  evidence  of  the  tendency  toward  saccus  for- 
mation. As  development  proceeds  in  the  selachian,  the  pituitary 
surface  maintains  its  primitive  relations  unchanged  while  the 
saccular  surface  becomes  more  extensive  and  in  Mustelus  at  the 
period  of  20  cm.  shows  the  first  evidence  of  a  rich  vascularization 
and-  the  tendency  of  its  thin  wall  to  be  thrown  into  numerous 
convolutions  typical  of  the  saccus  vasculosus.  The  pituitary 
and  saccular  surfaces  extend  laterad  for  a  considerable  distance, 
their  size  being  reduced  as  they  extend  farther  away  from 
the  median  line  so  that  they  ultimately  form  two  long  tapering 
outgrowths,  one  at  either  extremity  of  the  infundibular  process, 
called  the  lateral  processes.  Similar  outgrowths  are  observed 
in  the  development  of  the  bird,  and  while  no  definitely  correspond- 
ing structure  is  found  in  the  cat,  the  lateral  extremities  of  the 
infundibular  process  are  much  extended  in  a  manner  which 
seems  to  be  reminiscent  of  the  lateral  processes  in  the  bird  and 
selachian.  In  the  bird,  as  already  shown,  the  dorsal  surface 
of  the  infundibular  process  is  convoluted  but  non-vascular;  its 
walls  are  thick.  All  of  the  morphological  evidence  concerning 
it  tends  to  show  that  this  surface  is  the  strict  homologue  of  the 
saccular  surface  in  the  selachian,  and  that,  whereas  in  the  fish 
it  proceeds  to  form  the  saccus  vasculosus,  the  saccus  formation 
in  the  bird  is  aborted,  although  the  sauropsid  still  retains  evi- 
dence of  the  tendency  toward  the  formation  of  this  structure. 
Furthermore,  the  late  stages  of  embryonic  life  in  the  bird  corre- 
spond in  many  details  to  the  saccus  formation  as  it  appears  in 
this  region  of  the  selachian. 

In  the  mammal,  as  illustrated  by  the  cat  and  other  Felidae, 
there  is  nothing  which  suggests  the  saccus  formation  in  any  part 
of  the  infundibular  process  further  than  the  very  thin  dorsal 
surface  which  is  emphasized,  in  the  early  stages,  by  the  rapid 


THE    DIENCEPHALIC    FLOOR  273 

growth  and  thickness  in  the  ventral  wall  of  the  infundibular 
process.  This,  together  with  the  fact  that  in  the  Felidae  an  ex- 
tension of  the  third  ventricle  is  contained  in  the  infundibular 
process,  points  strongly  to  the  conclusion  that  the  dorsal  wall  of 
this  process  in  embryonic  stages  may  be  homologized  with  the 
saccular  surface  just  described  in  the  other  forms.  That  this 
area  in  mammals  becomes  invested  by  the  pars  infundibularis 
of  the  pituitary  gland,  may  be  interpreted  as  causing  the  retro- 
gression of  the  saccular  surface  and  its  replacement  by  a  new 
area  in  contact  with  tissue  of  the  pituitary  gland,  due  to  the 
greater  extension  of  the  infundibular  portion  of  this  gland  in 
mammals. 

Discussing  the  significance  of  the  saccus  vasculosus  and  its 
more  characteristic  formation  in  the  water-living  types  of  rep- 
tiles, Edinger  (18)  suggests  that  it  may  be  an  apparatus  of 
especial  importance  to  aquatic  animals.  This  inference,  how- 
ever interesting,  is  not  borne  out  by  the  facts  observed  in  some 
of  the  aquatic  mammals,  since  there  is  no  evidence  of  anything 
corresponding  to  the  saccus  vasculosus  or  even  an  abortive 
saccus-formation  in  Castor  canadensis  or  in  Macrorhinus  angus- 
tirostris.  The  absence,  therefore,  of  any  attempt  to  revert 
to  the  formation  of  a  saccus  vasculosus  in  mammals,  even 
though  they  be  water-living,  fails  to  corroborate  Edinger' s 
suggestion  that  this  apparatus  is  an  adaptation  peculiar  to 
aquatic  life.  It  seems  more  probable  that  the  disappearance 
of  the  saccus  vasculosus  depends  upon  a  profound  remodelling 
of  the  forebrain  which  occurs  in  passing  from  ichthyopsid  to 
the  sauropsid  and  mammalian  forms.  It  is  not  unlikely  that 
this  highly  vascularized  structure  is  closely  related  to  if  not 
identical  with  the  chorioid  formations  and  represents  in  the  fish 
a  means  of  supplying  an  extensive  chorioidal  plexus  to  the  third 
ventricle,  particularly  as  such  plexuses  are  relatively  small  in 
other  parts  of  the  diencephalon. 

The  dorsal  evagination  of  the  infundibular  region  still  remains 
to  be  considered.  As  already  stated,  this  evagination  in  the  cat 
is  of  greater  size  than  the  ventral  one  from  which  latter  the  in- 
fundibular process  is  derived.  In  the  bird  and  the  dog-fish 


274  FREDERICK   TILNEY 

the  reverse  is  true.  The  evagination  is  present  in  the  early 
stages  in  the  selachian  and  may  be  traced  through  to  adult  life, 
when  it  occupies  a  position  immediately  ventral  of  the  posterior 
lobe  and  dorsal  to  the  saccus  vasculosus.  Similarly  in  the 
bird  it  lies  dorsal  to  the  infundibular  process  from  its  inception 
and  maintains  this  position  in  the  adult.  Its  development  in 
the  cat  is  equally  clear;  here  it  forms  a  protuberance  of  the 
diencephalic  floor  caudal  to  the  infundibular  stem  and  in  front 
of  the  mammillary  bodies;  this  has  been  called  by  the  writer  the 
post-infundibular  eminence.  Much  discussion  has  arisen  con- 
cerning this  element  of  the  interbrain.  Retzius  (9),  who  first 
described  it  in  mammals,  considered  that  it  was  the  homologue 
of  the  saccus  vasculosus  in  fishes.  Staderini  (20),  however, 
states  that  topographically,  as  well  as  from  its  developmental 
relations,  the  saccus  vasculosus  can  bear  no  relation  to  the 
eminentia  saccularis.  It  is  also  his  opinion  that  nothing  in  the 
intimate  constitution  of  the  saccular  eminence  favors  the  con- 
ception of  Retzius.  In  fishes  the  saccus  vasculosus  is  a  thin- 
walled  structure  connected  with  a  great  number  of  blood  vessels. 
In  man  and  other  mammals  the  structure  referred  to  by  Retzius 
is  a  relatively  thick-walled  evagination  with  scanty  vasculari- 
zation.  Sterzi  (21)  is  unwilling  to  accept  the  interpretation  of 
Retzius  given  to  the  eminentia  saccularis,  and  believes  that  the 
saccus  vasculosus  has  nothing  in  common  with  the  latte'r  either 
in  position  or  structure,  while  Perna  (22)  on  histological  grounds 
maintains  that  there  can  be  no  homology  between  the  saccus 
vasculosus  and  the  so-called  'saccular  eminence.' 

The  post-infundibular  eminence  has  been  shown  in  illustration 
by  many  authors  in  a  number  of  different  species,  although 
it  has  not  always  beer  definitely  referred  to  in  their  text.  Herrick 
and  Obenchain  (23)  in  their  reconstruction  of  the  brain  of  Ichthy- 
omyzon  concolor  figure  a  small,  unleadered  eminence  ventrad  to 
the  mammillary  body,  in  which  there  appears  on  mid-sagittal  sec- 
tion a  small  recess  communicating  with  the  third  ventricle.  The 
only  reference  to  this  element  made  by  these  authors  is  the  atten- 
tion which  they  call  to  the  fact  that  the  post-infundibular  com- 
missure passes  through  the  more  cephalic  portion  of  the  structure. 


THE    DIENCEPHALIC    FLOOR  275 

In  surface  relief,  however,  it  forms  a  prominent  element  in  their 
illustration  of  the  diencephalic  floor  in  this  form.  Sterzi  (21) 
in  his  illustrations  of  Acanthias  vulgaris  (80  cm.  long),  in  Mus- 
telus  laevis  (30  cm.  long)  and  in  Raja  clavata  (60  cm.  long) 
shows  a  similar  structure  dorsal  to  the  saccus  vasculosus  and 
ventral  to  the  posterior  lobe  although  in  none  of  these  cases 
has  he  included  this  element  in  his  description.  Burckhardt 
(24)  shows  a  similar  condition  in  Protopterus  annectans.  In 
the  ontogenesis  of  the  dog-fish,  of  the  chick  and  of  the  cat  the 
dorsal  evagination  of  the  infundibular  region  is  a  constant  ele- 
ment and  may  be  traced  through  successive  stages  until  the 
definitive  post-infundibular  eminence  has  made  its  appearance. 
Thus  the  embryological  history  of  the  infundibular  region  seems 
to  make  clear  the  fact  that  the  inferior  lobes  may  be  homologized 
with  the  post-chiasmatic  eminence.  The  infundibular  process, 
including  as  it  does  the  saccus  vasculosus  of  the  ichthyopsid, 
is  the  homologue  of  the  infundibular  process  in  the  sauropsid 
and  mammal,  although  in  these  latter  forms  the  saccus  formation 
is  retrogressive  or  absent.  In  this  respect  the  writer  agrees  with 
Johnston  (11)  and  concurring  with  him  can  find  no  evidence  to 
support  Edinger's  (18)  idea  as  embodied  in  his  schematic  figure 
of  a  sagittal  section  of  the  vertebrate  brain  which  shows  an 
infundibular  process  in  contact  with  the  pituitary  gland  while 
dorsal  to  it  is  an  entirely  separate  evagination  of  the  brain  floor 
which  he  calls  the  saccus  vasculosus.  It  seems  equally  clear 
that  the  homology  of  the  post-infundibular  eminence  may  be 
established  throughout  the  phylum.  That  it  is  an  element 
separate  and  distinct  from  the  saccus  vasculosus  is  evident 
from  the  ontogeny  of  the  selachian  in  which  both  a  saccus  vas- 
culosus and  a  post-infundibular  eminence  are  present.  There 
can  be  no  grounds,  therefore,  for  the  homology  suggested  by 
Retzius  (9)  between  the  'saccular  eminence'  of  mammals  and 
the  saccus  vasculosus  of  fishes. 

Although  the  fact  has  not  been  fully  established,  the  evidence 
furnished  by  the  ontogenesis  of  the  dog-fish,  of  the  chick  and 
of  the  cat  strongly  suggests  that  the  derivatives  of  the  three 
segments  of  the  ectoptic  zone  are  coordinate.  In  this  light  the 


276  FREDERICK  TILNEY 

telencephalon  derived  from  the  cephalic  segment,  the  thala- 
mencephalon  from  the  dorsal  segment  and  the  infundibular 
region  from  the  ventral  segment  are  developmental  equivalents. 
It  has  been  shown  that  from  the  infundibular  region  in  the 
ichthyopsid  the  inferior  lobes,  the  infundibular  process  and  the 
post-infundibular  eminence  are  derived.  If,  as  has  been  assumed 
to  be  the  case  in  fishes,  the  inferior  lobes  are  chiefly  concerned 
in  the  gustatory  sense,  the  telencephalon  in  the  olfactory  sense 
and  the  thalamencephalon  in  the  somaesthetic  senses,  then 
there  is  further  reason  to  believe  that  the  derivatives  of  the  three 
segments  of  the  ectoptic  zone  are  functionally  of  the  same  order. 
The  disappearance  of  the  inferior  lobes  in  passing  from  the 
ichthyopsid  to  the  sauropsid  is,  as  a  process,  no  more  difficult 
to  comprehend  than  the  similar  disappearance  of  the  optic 
lobes  of  the  midbrain  in  the  transition  from  the  bird  to  the 
mammal.  In  each  instance  this  process  seems  to  be  accomplished 
by  the  addition  of  neopallial  areas  which  assume  the  functions 
of  the  more  primitive  brain  parts.  The  significance  of  the  per- 
sistence of  the  infundibular  process  and  the  post-infundibular 
eminence  is  less  clear,  although  in  the  former  case  this  doubtless 
is  involved  in  the  adaptive  variations  of  the  pituitary  gland, 
while  in  the  latter 'the  post-infundibular  commissure  may  be  a 
determinative  factor. 

The  tuber  cinereum  is  usually  described  as  an  area  in  the  basal 
region  of  the  diencephalon,  bounded  cephalad  by  the  optic  chiasm, 
caudad  by  the  mammillary  bodies  and  laterad  by  the  optic 
tracts  and  cerebral  peduncles.  This  area  in  mammals  includes 
the  post-chiasmatic  eminence,  the  lateral  eminences  and  the  post- 
infundibular  eminence.  From  the  embryological  standpoint, 
the  tuber  cinereum  comprises  all  of  the  derivatives  of  the  in- 
fundibular region  except  the  infundibular  process  and  its  stem. 
To  hold  the  tuber  to  this  interpretation  in  fishes  and  amphibia 
would  necessitate  the  inclusion  in  it  of  the  inferior  lobes,  but 
since  this  has  no  apparent  advantage  in  the  lower  forms  men- 
tioned it  is,  perhaps,  well  to  confine  the  term  tuber  cinereum  to 
mammals,  in  which  instances  it  is  useful  in  referring  to  a  dis- 
tinctive region  of  the  diencephalic  floor.  The  homology  of  the 


THE    DIENCEPHALIC    FLOOR  277 

lateral  eminences  of  the  tuber  is  not  clear  in  the  light  of  the 
material  studied.  Conditions  in  the  teleost,  however,  in  which 
the  hypoarium  presents  small  lobi  inferiores  and  much  larger 
lobi  laterales,  may  be  regarded  as  suggestive,  since  the  progressive 
reduction  of  the  inferior  lobes  has  been  shown  to  result  in  the 
formation  of  the  post-chiasmatic  eminence  and  a  similar  diminu- 
tion of  the  lateral  lobes  might,  therefore,  determine  the  emi- 
nentiae  laterales  hypencephali.  This  homology  is  tentatively 
offered,  since  it  requires  further  proof  in  the  development  of 
the  teleost  to  establish  it. 

The  mammillary  region.  The  posterior  lobe  is  the  most  caudal 
structure  in  the  diencephalic  floor  of  the  selachian.  Its  relations 
have  already  been  described  (page  231).  It  consists  of  a  median 
portion  and  two  lateral  processes  which  project  free,  one  upon 
either  side.  Its  characteristic  feature  appears  in  the  fact  that 
it  contains  a  large  recess  of  the  third  ventricle  which  extends 
from  the  median  portion  into  the  two  lateral  processes.  Sterzi 
(21)  has  figured  and  described  the  posterior  lobe  in  selachians; 
von  Kuppfer  (7)  has  shown  it  in  Bellostoma,  Squalus  acanthius, 
and  Xecturus.  Edinger  (18)  describes  it  in  the  selachian  as 
the  lobus  posterior  sive  saccus  infundibuli.  Johnston  (11) 
shows  the  posterior  lobe  as  well  as  the  post-infundibular  eminence 
in  a  figure  of  the  mesial  surface  of  the  right  half  of  the  brain  in 
Squalus  acanthius,  although  neither  of  these  structures  is  spe- 
cifically named  by  him  in  this  place.  Herrick  and  Obenchain 
(23),  in  their  illustration  taken  from  the  reconstruction  of  the 
brain  in  Ichthyomyzon  concolor,  indicate  a  structure  similar 
in  relations  and  characteristics  to  the  posterior  lobe  which  they 
call  the  corpus  mammillare.  In  several  respects  the  designation 
given  the  structure  by  the  last  named  authors  seems  to  be  most 
in  keeping  with  the  facts,  for  although  the  majority  of  investi- 
gators have  employed  the  term  posterior  lobe  in  selachian  and 
teleosts,  the  embryological  history  of  the  structure  clearly  shows 
that  it  is  derived  from  the  primitive  mammillary  region.  In 
the  ichthyopsid  the  mammillary  region  develops  in  such  a  way 
as  to  form  a  posterior  lobe  presenting  the  characteristics  already 
described  and  retaining  a  recess  accessory  to  the  third  ventricle, 


278  FREDERICK   TILNEY 

the  recessus  lobi  posterioris.  In  birds  and  reptiles  the  develop- 
mental history  of  the  mammillary  region  through  the  early 
stages  is  similar  to  that  in  the  fish.  Later,  however,  the  walls 
of  this  region  begin  to  thicken  rapidly  and  the  mammillary 
recess  becomes  progressively  reduced  in  size  until  it  is  obliterated 
and  the  solid  mammillary  bodies  have  been  formed.  The 
marked  cephalic  flexure  in  the  fowl  causes  a  divergence  in  the 
long  axes  of  the  mammillary  bodies  caudo-cephalad,  so  that 
these  structures  do  not  present  the  same  prominence  here  as 
they  do  in  the  diencephalic  floor  of  mammals.  The  develop- 
ment of  the  mammillary  region  in  the  cat  manifests  certain 
peculiarities  which  I  was  unable  to  observe  in  either  the  chick 
or  the  dog-fish.  These  peculiarities  appear  in  the  formation 
of  two  relatively  early  diverticula,  the  median  mammillary 
evagination  and  the  dorsal  mammillary  evagination.  The 
latter  evagination  is  unquestionably  involved  in  the  formation 
of  the  corpus  interpedunculare,  for  I  have  found  that  the  fascic- 
ulus retroflexus  of  Meynert  may  be  seen  passing  from  the 
habenular  region  directly  to  the  evagination  in  question  as  early 
as  the  stage  of  25  mm.  There  can  be  no  doubt  that  this  dorsal 
evagination,  therefore,  is  the  anlage  of  the  corpus  interpeduncu- 
lare. That  no  similar  evagination  has  been  found  either  in  the 
dog-fish  or  in  the  chick  may  argue  that  a  less  definite  portion 
of  the  mammillary  region  gives  rise  to  the  corpus  interpedunculare 
in  these  forms,  but  it  is  probable  that  this  ganglionic  body  takes 
origin  from  the  primitive  mammillary  region,  even  though  no 
distinct  evagination  of  its  own  is  formed.  To  establish  this 
supposition,  however,  it  will  be  necessary  to  study  the  develop- 
ment of  this  region  further,  particularly  with  a  view  to  the 
ontogenesis  of  the  fiber  tracts  connecting  the  several  centers 
involved.  In  the  light  of  these  facts,  with  the  exception  of  this 
interpeduncular  element,  it  seems  warranted  to  homologize 
the  ichthyopsid  posterior  lobe  with  the  mammillary  bodies  of 
sauropsids  and  mammals. 


THE    DIENCEPHALIC    FLOOR  279 

CONCLUSIONS 

The  supraoptic  crest,  chiasmatic  process,  prechiasmatic  and 
supraoptic  recesses  in  the  mammal  have  their  definite  homologues 
in  the  sauropsid  and  ichthyopsid. 

Of  the  structures  derived  from  the  ventral  segment  of  the 
ectoptic  zone  the  post-chiasmatic  eminence  of  the  mammal  and 
bird  may  be  homologized  with  the  inferior  lobes  or  hypoarium 
of  fishes,  while  there  is  some  evidence  which  seems  to  indicate 
that  the  eminentiae  laterales  hypencephali  are  the  homologues 
of  the  lateral  lobes  of  teleosts. 

The  derivatives  of  the  caudal  portion  of  the  infundibular  region, 
including  its  apex,  are  the  infundibular  process  and  post-infundib- 
ular eminence. 

The  infundibular  process  in  the  selachian  presents  a  pituitary 
and  a  saccular  surface,  the  latter  forming  the  saccus  vasculosus. 
In  the  bird  these  two  surfaces  are  present;  the  saccular  surface, 
although  it  has  some  of  the  characteristics  of  a  saccus-formation, 
does  not  present  an  actual  saccus  vasculosus.  So  far  as  may  now 
be  stated  for  the  condition  in  mammals,  the  Felidae  present  an  ex- 
tensive pituitary  surface  in  their  infundibular  process.  The  saccu- 
lar surface,  however,  has  lost  all  characteristics  of  saccus-formation 
and  is  in  fact  invested  by  tissue  of  the  pituitary  gland.  In  other 
mammals  it  is  difficult  to  draw  distinction  between  the  pituitary 
and  saccular  surfaces  of  the  infundibular  process,  and  this 
differentiation  in  Mammalia  would  indeed  be  impossible  were 
it  not  for  the  intermediate  position  of  the  Felidae  in  this  respect 
between  the  bird,  on  the  one  hand,  and  the  majority  of  mammals 
on  the  other.  The  lateral  extensions  of  the  infundibular  process, 
the  infundibular  recess,  infundibular  stem  and  infundibular 
canal  of  the  domestic  cat  all  have  their  homologues  in  the  bird 
and  selachian.  The  disappearance  of  the  saccus  vasculosus 
from  the  mammal  may  be  traced  through  several  stages  of 
retrogression  from  the  dog-fish  to  the  cat,  so  that  the  homology 
of  the  infundibular  process  as  a  whole  in  the  mammal  with  that 
of  the  selachian  seems  to  be  warrantable. 

THE  JOURNAL  OP  COMPARATIVE  NEUROLOGY,  VOL.  25,  NO.  3 


280  FREDERICK   TILNEY 

The  post-infundibular  eminence  of  the  mammal  seems  to  bear 
a  clear  homology  to  that  of  the  bird  and  selachian.  The  embryo- 
logical  evidence  concerning  the  development  of  this  region  is 
strongly  suggestive  if  not  conclusive  in  establishing  this  homol- 
ogy. The  same  facts  make  it  impossible  to  consider  the  post- 
infundibular  eminence  as  phylogenetically  related  to  the  saccus 
vasculosus,  and  hence  raise  serious  objection  to  the  term 
'eminentia  saccularis'  as  applied  to  it. 

There  can  be  little  doubt  that  the  posterior  lobe  of  the  se- 
lachian is  the  homologue  of  the  mammillary  body  in  the  bird 
and  mammal. 


THE    DIENCEPHALIC    FLOOR  281 


LITERATURE  CITED 

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(3)  GANIN,  M.    1870    Zeitschrift  fur  w.  Zoolog.,  Bd.  20. 

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(10)  His,  W.    1895    Die  Anatomische  Nomenclature.    Leipzig. 

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(12)  SCHULTE,  H.  W.,  and  TILNEY,  F.    1915    The   development  of  the  neu- 

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(13)  OSBORN,  H.  F.    1888    Contribution  to  the  anatomy  of  the  central  nervous 

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lanceolatus.    Anat.  Anz.,  Bd.  21,  page  411. 


282  FREDERICK   TILNEY 

(20)  STADERINI,  R.     1909    Intorno  alia  eminentia  saccularis  ed  al  suo  signif- 

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